Mapping Class Inheritance Hierarchies¶
SQLAlchemy supports three forms of inheritance: single table inheritance, where several types of classes are represented by a single table, concrete table inheritance, where each type of class is represented by independent tables, and joined table inheritance, where the class hierarchy is broken up among dependent tables, each class represented by its own table that only includes those attributes local to that class.
The most common forms of inheritance are single and joined table, while concrete inheritance presents more configurational challenges.
When mappers are configured in an inheritance relationship, SQLAlchemy has the ability to load elements polymorphically, meaning that a single query can return objects of multiple types.
See also
Inheritance Mapping Recipes - complete examples of joined, single and concrete inheritance
Joined Table Inheritance¶
In joined table inheritance, each class along a hierarchy of classes is represented by a distinct table. Querying for a particular subclass in the hierarchy will render as a SQL JOIN along all tables in its inheritance path. If the queried class is the base class, the default behavior is to include only the base table in a SELECT statement. In all cases, the ultimate class to instantiate for a given row is determined by a discriminator column or an expression that works against the base table. When a subclass is loaded only against a base table, resulting objects will have base attributes populated at first; attributes that are local to the subclass will lazy load when they are accessed. Alternatively, there are options which can change the default behavior, allowing the query to include columns corresponding to multiple tables/subclasses up front.
The base class in a joined inheritance hierarchy is configured with additional arguments that will refer to the polymorphic discriminator column as well as the identifier for the base class:
class Employee(Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
type = Column(String(50))
__mapper_args__ = {
"polymorphic_identity": "employee",
"polymorphic_on": type,
}
Above, an additional column type
is established to act as the
discriminator, configured as such using the mapper.polymorphic_on
parameter. This column will store a value which indicates the type of object
represented within the row. The column may be of any datatype, though string
and integer are the most common. The actual data value to be applied to this
column for a particular row in the database is specified using the
mapper.polymorphic_identity
parameter, described below.
While a polymorphic discriminator expression is not strictly necessary, it is required if polymorphic loading is desired. Establishing a simple column on the base table is the easiest way to achieve this, however very sophisticated inheritance mappings may even configure a SQL expression such as a CASE statement as the polymorphic discriminator.
Note
Currently, only one discriminator column or SQL expression may be configured for the entire inheritance hierarchy, typically on the base- most class in the hierarchy. “Cascading” polymorphic discriminator expressions are not yet supported.
We next define Engineer
and Manager
subclasses of Employee
.
Each contains columns that represent the attributes unique to the subclass
they represent. Each table also must contain a primary key column (or
columns), as well as a foreign key reference to the parent table:
class Engineer(Employee):
__tablename__ = "engineer"
id = Column(Integer, ForeignKey("employee.id"), primary_key=True)
engineer_name = Column(String(30))
__mapper_args__ = {
"polymorphic_identity": "engineer",
}
class Manager(Employee):
__tablename__ = "manager"
id = Column(Integer, ForeignKey("employee.id"), primary_key=True)
manager_name = Column(String(30))
__mapper_args__ = {
"polymorphic_identity": "manager",
}
In the above example, each mapping specifies the
mapper.polymorphic_identity
parameter within its mapper arguments.
This value populates the column designated by the
mapper.polymorphic_on
parameter established on the base mapper.
The mapper.polymorphic_identity
parameter should be unique to
each mapped class across the whole hierarchy, and there should only be one
“identity” per mapped class; as noted above, “cascading” identities where some
subclasses introduce a second identity are not supported.
The ORM uses the value set up by mapper.polymorphic_identity
in
order to determine which class a row belongs towards when loading rows
polymorphically. In the example above, every row which represents an
Employee
will have the value 'employee'
in its type
row; similarly,
every Engineer
will get the value 'engineer'
, and each Manager
will
get the value 'manager'
. Regardless of whether the inheritance mapping uses
distinct joined tables for subclasses as in joined table inheritance, or all
one table as in single table inheritance, this value is expected to be
persisted and available to the ORM when querying. The
mapper.polymorphic_identity
parameter also applies to concrete
table inheritance, but is not actually persisted; see the later section at
Concrete Table Inheritance for details.
In a polymorphic setup, it is most common that the foreign key constraint is established on the same column or columns as the primary key itself, however this is not required; a column distinct from the primary key may also be made to refer to the parent via foreign key. The way that a JOIN is constructed from the base table to subclasses is also directly customizable, however this is rarely necessary.
With the joined inheritance mapping complete, querying against Employee
will return a combination of Employee
, Engineer
and Manager
objects. Newly saved Engineer
, Manager
, and Employee
objects will
automatically populate the employee.type
column with the correct
“discriminator” value in this case "engineer"
,
"manager"
, or "employee"
, as appropriate.
Relationships with Joined Inheritance¶
Relationships are fully supported with joined table inheritance. The
relationship involving a joined-inheritance class should target the class
in the hierarchy that also corresponds to the foreign key constraint;
below, as the employee
table has a foreign key constraint back to
the company
table, the relationships are set up between Company
and Employee
:
class Company(Base):
__tablename__ = "company"
id = Column(Integer, primary_key=True)
name = Column(String(50))
employees = relationship("Employee", back_populates="company")
class Employee(Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
type = Column(String(50))
company_id = Column(ForeignKey("company.id"))
company = relationship("Company", back_populates="employees")
__mapper_args__ = {
"polymorphic_identity": "employee",
"polymorphic_on": type,
}
class Manager(Employee):
...
class Engineer(Employee):
...
If the foreign key constraint is on a table corresponding to a subclass,
the relationship should target that subclass instead. In the example
below, there is a foreign
key constraint from manager
to company
, so the relationships are
established between the Manager
and Company
classes:
class Company(Base):
__tablename__ = "company"
id = Column(Integer, primary_key=True)
name = Column(String(50))
managers = relationship("Manager", back_populates="company")
class Employee(Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
type = Column(String(50))
__mapper_args__ = {
"polymorphic_identity": "employee",
"polymorphic_on": type,
}
class Manager(Employee):
__tablename__ = "manager"
id = Column(Integer, ForeignKey("employee.id"), primary_key=True)
manager_name = Column(String(30))
company_id = Column(ForeignKey("company.id"))
company = relationship("Company", back_populates="managers")
__mapper_args__ = {
"polymorphic_identity": "manager",
}
class Engineer(Employee):
...
Above, the Manager
class will have a Manager.company
attribute;
Company
will have a Company.managers
attribute that always
loads against a join of the employee
and manager
tables together.
Loading Joined Inheritance Mappings¶
See the sections Loading Inheritance Hierarchies and Loading objects with joined table inheritance for background on inheritance loading techniques, including configuration of tables to be queried both at mapper configuration time as well as query time.
Single Table Inheritance¶
Single table inheritance represents all attributes of all subclasses within a single table. A particular subclass that has attributes unique to that class will persist them within columns in the table that are otherwise NULL if the row refers to a different kind of object.
Querying for a particular subclass in the hierarchy will render as a SELECT against the base table, which will include a WHERE clause that limits rows to those with a particular value or values present in the discriminator column or expression.
Single table inheritance has the advantage of simplicity compared to joined table inheritance; queries are much more efficient as only one table needs to be involved in order to load objects of every represented class.
Single-table inheritance configuration looks much like joined-table
inheritance, except only the base class specifies __tablename__
. A
discriminator column is also required on the base table so that classes can be
differentiated from each other.
Even though subclasses share the base table for all of their attributes,
when using Declarative, Column
objects may still be specified on
subclasses, indicating that the column is to be mapped only to that subclass;
the Column
will be applied to the same base Table
object:
class Employee(Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
type = Column(String(20))
__mapper_args__ = {
"polymorphic_on": type,
"polymorphic_identity": "employee",
}
class Manager(Employee):
manager_data = Column(String(50))
__mapper_args__ = {
"polymorphic_identity": "manager",
}
class Engineer(Employee):
engineer_info = Column(String(50))
__mapper_args__ = {
"polymorphic_identity": "engineer",
}
Note that the mappers for the derived classes Manager and Engineer omit the
__tablename__
, indicating they do not have a mapped table of
their own.
Resolving Column Conflicts¶
Note in the previous section that the manager_name
and engineer_info
columns
are “moved up” to be applied to Employee.__table__
, as a result of their
declaration on a subclass that has no table of its own. A tricky case
comes up when two subclasses want to specify the same column, as below:
class Employee(Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
type = Column(String(20))
__mapper_args__ = {
"polymorphic_on": type,
"polymorphic_identity": "employee",
}
class Engineer(Employee):
__mapper_args__ = {
"polymorphic_identity": "engineer",
}
start_date = Column(DateTime)
class Manager(Employee):
__mapper_args__ = {
"polymorphic_identity": "manager",
}
start_date = Column(DateTime)
Above, the start_date
column declared on both Engineer
and Manager
will result in an error:
sqlalchemy.exc.ArgumentError: Column 'start_date' on class
<class '__main__.Manager'> conflicts with existing
column 'employee.start_date'
The above scenario presents an ambiguity to the Declarative mapping system that
may be resolved by using
declared_attr
to define the Column
conditionally,
taking care to return the existing column via the parent __table__
if it already exists:
from sqlalchemy.orm import declared_attr
class Employee(Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
type = Column(String(20))
__mapper_args__ = {
"polymorphic_on": type,
"polymorphic_identity": "employee",
}
class Engineer(Employee):
__mapper_args__ = {
"polymorphic_identity": "engineer",
}
@declared_attr
def start_date(cls):
"Start date column, if not present already."
return Employee.__table__.c.get("start_date", Column(DateTime))
class Manager(Employee):
__mapper_args__ = {
"polymorphic_identity": "manager",
}
@declared_attr
def start_date(cls):
"Start date column, if not present already."
return Employee.__table__.c.get("start_date", Column(DateTime))
Above, when Manager
is mapped, the start_date
column is
already present on the Employee
class; by returning the existing
Column
object, the declarative system recognizes that this
is the same column to be mapped to the two different subclasses separately.
A similar concept can be used with mixin classes (see Composing Mapped Hierarchies with Mixins) to define a particular series of columns and/or other mapped attributes from a reusable mixin class:
class Employee(Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
type = Column(String(20))
__mapper_args__ = {
"polymorphic_on": type,
"polymorphic_identity": "employee",
}
class HasStartDate:
@declared_attr
def start_date(cls):
return cls.__table__.c.get("start_date", Column(DateTime))
class Engineer(HasStartDate, Employee):
__mapper_args__ = {
"polymorphic_identity": "engineer",
}
class Manager(HasStartDate, Employee):
__mapper_args__ = {
"polymorphic_identity": "manager",
}
Relationships with Single Table Inheritance¶
Relationships are fully supported with single table inheritance. Configuration is done in the same manner as that of joined inheritance; a foreign key attribute should be on the same class that’s the “foreign” side of the relationship:
class Company(Base):
__tablename__ = "company"
id = Column(Integer, primary_key=True)
name = Column(String(50))
employees = relationship("Employee", back_populates="company")
class Employee(Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
type = Column(String(50))
company_id = Column(ForeignKey("company.id"))
company = relationship("Company", back_populates="employees")
__mapper_args__ = {
"polymorphic_identity": "employee",
"polymorphic_on": type,
}
class Manager(Employee):
manager_data = Column(String(50))
__mapper_args__ = {
"polymorphic_identity": "manager",
}
class Engineer(Employee):
engineer_info = Column(String(50))
__mapper_args__ = {
"polymorphic_identity": "engineer",
}
Also, like the case of joined inheritance, we can create relationships that involve a specific subclass. When queried, the SELECT statement will include a WHERE clause that limits the class selection to that subclass or subclasses:
class Company(Base):
__tablename__ = "company"
id = Column(Integer, primary_key=True)
name = Column(String(50))
managers = relationship("Manager", back_populates="company")
class Employee(Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
type = Column(String(50))
__mapper_args__ = {
"polymorphic_identity": "employee",
"polymorphic_on": type,
}
class Manager(Employee):
manager_name = Column(String(30))
company_id = Column(ForeignKey("company.id"))
company = relationship("Company", back_populates="managers")
__mapper_args__ = {
"polymorphic_identity": "manager",
}
class Engineer(Employee):
engineer_info = Column(String(50))
__mapper_args__ = {
"polymorphic_identity": "engineer",
}
Above, the Manager
class will have a Manager.company
attribute;
Company
will have a Company.managers
attribute that always
loads against the employee
with an additional WHERE clause that
limits rows to those with type = 'manager'
.
Loading Single Inheritance Mappings¶
The loading techniques for single-table inheritance are mostly identical to
those used for joined-table inheritance, and a high degree of abstraction is
provided between these two mapping types such that it is easy to switch between
them as well as to intermix them in a single hierarchy (just omit
__tablename__
from whichever subclasses are to be single-inheriting). See
the sections Loading Inheritance Hierarchies and
Loading objects with single table inheritance for documentation on inheritance loading
techniques, including configuration of classes to be queried both at mapper
configuration time as well as query time.
Concrete Table Inheritance¶
Concrete inheritance maps each subclass to its own distinct table, each of which contains all columns necessary to produce an instance of that class. A concrete inheritance configuration by default queries non-polymorphically; a query for a particular class will only query that class’ table and only return instances of that class. Polymorphic loading of concrete classes is enabled by configuring within the mapper a special SELECT that typically is produced as a UNION of all the tables.
Warning
Concrete table inheritance is much more complicated than joined or single table inheritance, and is much more limited in functionality especially pertaining to using it with relationships, eager loading, and polymorphic loading. When used polymorphically it produces very large queries with UNIONS that won’t perform as well as simple joins. It is strongly advised that if flexibility in relationship loading and polymorphic loading is required, that joined or single table inheritance be used if at all possible. If polymorphic loading isn’t required, then plain non-inheriting mappings can be used if each class refers to its own table completely.
Whereas joined and single table inheritance are fluent in “polymorphic” loading, it is a more awkward affair in concrete inheritance. For this reason, concrete inheritance is more appropriate when polymorphic loading is not required. Establishing relationships that involve concrete inheritance classes is also more awkward.
To establish a class as using concrete inheritance, add the
mapper.concrete
parameter within the __mapper_args__
.
This indicates to Declarative as well as the mapping that the superclass
table should not be considered as part of the mapping:
class Employee(Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
class Manager(Employee):
__tablename__ = "manager"
id = Column(Integer, primary_key=True)
name = Column(String(50))
manager_data = Column(String(50))
__mapper_args__ = {
"concrete": True,
}
class Engineer(Employee):
__tablename__ = "engineer"
id = Column(Integer, primary_key=True)
name = Column(String(50))
engineer_info = Column(String(50))
__mapper_args__ = {
"concrete": True,
}
Two critical points should be noted:
We must define all columns explicitly on each subclass, even those of the same name. A column such as
Employee.name
here is not copied out to the tables mapped byManager
orEngineer
for us.while the
Engineer
andManager
classes are mapped in an inheritance relationship withEmployee
, they still do not include polymorphic loading. Meaning, if we query forEmployee
objects, themanager
andengineer
tables are not queried at all.
Concrete Polymorphic Loading Configuration¶
Polymorphic loading with concrete inheritance requires that a specialized
SELECT is configured against each base class that should have polymorphic
loading. This SELECT needs to be capable of accessing all the
mapped tables individually, and is typically a UNION statement that is
constructed using a SQLAlchemy helper polymorphic_union()
.
As discussed in Loading Inheritance Hierarchies, mapper inheritance
configurations of any type can be configured to load from a special selectable
by default using the mapper.with_polymorphic
argument. Current
public API requires that this argument is set on a Mapper
when
it is first constructed.
However, in the case of Declarative, both the mapper and the Table
that is mapped are created at once, the moment the mapped class is defined.
This means that the mapper.with_polymorphic
argument cannot
be provided yet, since the Table
objects that correspond to the
subclasses haven’t yet been defined.
There are a few strategies available to resolve this cycle, however
Declarative provides helper classes ConcreteBase
and
AbstractConcreteBase
which handle this issue behind the scenes.
Using ConcreteBase
, we can set up our concrete mapping in
almost the same way as we do other forms of inheritance mappings:
from sqlalchemy.ext.declarative import ConcreteBase
from sqlalchemy.orm import declarative_base
Base = declarative_base()
class Employee(ConcreteBase, Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
__mapper_args__ = {
"polymorphic_identity": "employee",
"concrete": True,
}
class Manager(Employee):
__tablename__ = "manager"
id = Column(Integer, primary_key=True)
name = Column(String(50))
manager_data = Column(String(40))
__mapper_args__ = {
"polymorphic_identity": "manager",
"concrete": True,
}
class Engineer(Employee):
__tablename__ = "engineer"
id = Column(Integer, primary_key=True)
name = Column(String(50))
engineer_info = Column(String(40))
__mapper_args__ = {
"polymorphic_identity": "engineer",
"concrete": True,
}
Above, Declarative sets up the polymorphic selectable for the
Employee
class at mapper “initialization” time; this is the late-configuration
step for mappers that resolves other dependent mappers. The ConcreteBase
helper uses the
polymorphic_union()
function to create a UNION of all concrete-mapped
tables after all the other classes are set up, and then configures this statement
with the already existing base-class mapper.
Upon select, the polymorphic union produces a query like this:
session.query(Employee).all()
SELECT
pjoin.id AS pjoin_id,
pjoin.name AS pjoin_name,
pjoin.type AS pjoin_type,
pjoin.manager_data AS pjoin_manager_data,
pjoin.engineer_info AS pjoin_engineer_info
FROM (
SELECT
employee.id AS id,
employee.name AS name,
CAST(NULL AS VARCHAR(50)) AS manager_data,
CAST(NULL AS VARCHAR(50)) AS engineer_info,
'employee' AS type
FROM employee
UNION ALL
SELECT
manager.id AS id,
manager.name AS name,
manager.manager_data AS manager_data,
CAST(NULL AS VARCHAR(50)) AS engineer_info,
'manager' AS type
FROM manager
UNION ALL
SELECT
engineer.id AS id,
engineer.name AS name,
CAST(NULL AS VARCHAR(50)) AS manager_data,
engineer.engineer_info AS engineer_info,
'engineer' AS type
FROM engineer
) AS pjoin
The above UNION query needs to manufacture “NULL” columns for each subtable in order to accommodate for those columns that aren’t members of that particular subclass.
See also
ConcreteBase
Abstract Concrete Classes¶
The concrete mappings illustrated thus far show both the subclasses as well as the base class mapped to individual tables. In the concrete inheritance use case, it is common that the base class is not represented within the database, only the subclasses. In other words, the base class is “abstract”.
Normally, when one would like to map two different subclasses to individual
tables, and leave the base class unmapped, this can be achieved very easily.
When using Declarative, just declare the
base class with the __abstract__
indicator:
from sqlalchemy.orm import declarative_base
Base = declarative_base()
class Employee(Base):
__abstract__ = True
class Manager(Employee):
__tablename__ = "manager"
id = Column(Integer, primary_key=True)
name = Column(String(50))
manager_data = Column(String(40))
class Engineer(Employee):
__tablename__ = "engineer"
id = Column(Integer, primary_key=True)
name = Column(String(50))
engineer_info = Column(String(40))
Above, we are not actually making use of SQLAlchemy’s inheritance mapping
facilities; we can load and persist instances of Manager
and Engineer
normally. The situation changes however when we need to query polymorphically,
that is, we’d like to emit session.query(Employee)
and get back a collection
of Manager
and Engineer
instances. This brings us back into the
domain of concrete inheritance, and we must build a special mapper against
Employee
in order to achieve this.
To modify our concrete inheritance example to illustrate an “abstract” base
that is capable of polymorphic loading,
we will have only an engineer
and a manager
table and no employee
table, however the Employee
mapper will be mapped directly to the
“polymorphic union”, rather than specifying it locally to the
mapper.with_polymorphic
parameter.
To help with this, Declarative offers a variant of the ConcreteBase
class called AbstractConcreteBase
which achieves this automatically:
from sqlalchemy.ext.declarative import AbstractConcreteBase
from sqlalchemy.orm import declarative_base
Base = declarative_base()
class Employee(AbstractConcreteBase, Base):
pass
class Manager(Employee):
__tablename__ = "manager"
id = Column(Integer, primary_key=True)
name = Column(String(50))
manager_data = Column(String(40))
__mapper_args__ = {
"polymorphic_identity": "manager",
"concrete": True,
}
class Engineer(Employee):
__tablename__ = "engineer"
id = Column(Integer, primary_key=True)
name = Column(String(50))
engineer_info = Column(String(40))
__mapper_args__ = {
"polymorphic_identity": "engineer",
"concrete": True,
}
Base.registry.configure()
Above, the registry.configure()
method is invoked, which will
trigger the Employee
class to be actually mapped; before the configuration
step, the class has no mapping as the sub-tables which it will query from
have not yet been defined. This process is more complex than that of
ConcreteBase
, in that the entire mapping
of the base class must be delayed until all the subclasses have been declared.
With a mapping like the above, only instances of Manager
and Engineer
may be persisted; querying against the Employee
class will always produce
Manager
and Engineer
objects.
See also
AbstractConcreteBase
Classical and Semi-Classical Concrete Polymorphic Configuration¶
The Declarative configurations illustrated with ConcreteBase
and AbstractConcreteBase
are equivalent to two other forms
of configuration that make use of polymorphic_union()
explicitly.
These configurational forms make use of the Table
object explicitly
so that the “polymorphic union” can be created first, then applied
to the mappings. These are illustrated here to clarify the role
of the polymorphic_union()
function in terms of mapping.
A semi-classical mapping for example makes use of Declarative, but
establishes the Table
objects separately:
metadata_obj = Base.metadata
employees_table = Table(
"employee",
metadata_obj,
Column("id", Integer, primary_key=True),
Column("name", String(50)),
)
managers_table = Table(
"manager",
metadata_obj,
Column("id", Integer, primary_key=True),
Column("name", String(50)),
Column("manager_data", String(50)),
)
engineers_table = Table(
"engineer",
metadata_obj,
Column("id", Integer, primary_key=True),
Column("name", String(50)),
Column("engineer_info", String(50)),
)
Next, the UNION is produced using polymorphic_union()
:
from sqlalchemy.orm import polymorphic_union
pjoin = polymorphic_union(
{
"employee": employees_table,
"manager": managers_table,
"engineer": engineers_table,
},
"type",
"pjoin",
)
With the above Table
objects, the mappings can be produced using “semi-classical” style,
where we use Declarative in conjunction with the __table__
argument;
our polymorphic union above is passed via __mapper_args__
to
the mapper.with_polymorphic
parameter:
class Employee(Base):
__table__ = employee_table
__mapper_args__ = {
"polymorphic_on": pjoin.c.type,
"with_polymorphic": ("*", pjoin),
"polymorphic_identity": "employee",
}
class Engineer(Employee):
__table__ = engineer_table
__mapper_args__ = {
"polymorphic_identity": "engineer",
"concrete": True,
}
class Manager(Employee):
__table__ = manager_table
__mapper_args__ = {
"polymorphic_identity": "manager",
"concrete": True,
}
Alternatively, the same Table
objects can be used in
fully “classical” style, without using Declarative at all.
A constructor similar to that supplied by Declarative is illustrated:
class Employee(object):
def __init__(self, **kw):
for k in kw:
setattr(self, k, kw[k])
class Manager(Employee):
pass
class Engineer(Employee):
pass
employee_mapper = mapper_registry.map_imperatively(
Employee,
pjoin,
with_polymorphic=("*", pjoin),
polymorphic_on=pjoin.c.type,
)
manager_mapper = mapper_registry.map_imperatively(
Manager,
managers_table,
inherits=employee_mapper,
concrete=True,
polymorphic_identity="manager",
)
engineer_mapper = mapper_registry.map_imperatively(
Engineer,
engineers_table,
inherits=employee_mapper,
concrete=True,
polymorphic_identity="engineer",
)
The “abstract” example can also be mapped using “semi-classical” or “classical”
style. The difference is that instead of applying the “polymorphic union”
to the mapper.with_polymorphic
parameter, we apply it directly
as the mapped selectable on our basemost mapper. The semi-classical
mapping is illustrated below:
from sqlalchemy.orm import polymorphic_union
pjoin = polymorphic_union(
{
"manager": managers_table,
"engineer": engineers_table,
},
"type",
"pjoin",
)
class Employee(Base):
__table__ = pjoin
__mapper_args__ = {
"polymorphic_on": pjoin.c.type,
"with_polymorphic": "*",
"polymorphic_identity": "employee",
}
class Engineer(Employee):
__table__ = engineer_table
__mapper_args__ = {
"polymorphic_identity": "engineer",
"concrete": True,
}
class Manager(Employee):
__table__ = manager_table
__mapper_args__ = {
"polymorphic_identity": "manager",
"concrete": True,
}
Above, we use polymorphic_union()
in the same manner as before, except
that we omit the employee
table.
See also
Imperative Mapping - background information on imperative, or “classical” mappings
Relationships with Concrete Inheritance¶
In a concrete inheritance scenario, mapping relationships is challenging
since the distinct classes do not share a table. If the relationships
only involve specific classes, such as a relationship between Company
in
our previous examples and Manager
, special steps aren’t needed as these
are just two related tables.
However, if Company
is to have a one-to-many relationship
to Employee
, indicating that the collection may include both
Engineer
and Manager
objects, that implies that Employee
must
have polymorphic loading capabilities and also that each table to be related
must have a foreign key back to the company
table. An example of
such a configuration is as follows:
from sqlalchemy.ext.declarative import ConcreteBase
class Company(Base):
__tablename__ = "company"
id = Column(Integer, primary_key=True)
name = Column(String(50))
employees = relationship("Employee")
class Employee(ConcreteBase, Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
company_id = Column(ForeignKey("company.id"))
__mapper_args__ = {
"polymorphic_identity": "employee",
"concrete": True,
}
class Manager(Employee):
__tablename__ = "manager"
id = Column(Integer, primary_key=True)
name = Column(String(50))
manager_data = Column(String(40))
company_id = Column(ForeignKey("company.id"))
__mapper_args__ = {
"polymorphic_identity": "manager",
"concrete": True,
}
class Engineer(Employee):
__tablename__ = "engineer"
id = Column(Integer, primary_key=True)
name = Column(String(50))
engineer_info = Column(String(40))
company_id = Column(ForeignKey("company.id"))
__mapper_args__ = {
"polymorphic_identity": "engineer",
"concrete": True,
}
The next complexity with concrete inheritance and relationships involves
when we’d like one or all of Employee
, Manager
and Engineer
to
themselves refer back to Company
. For this case, SQLAlchemy has
special behavior in that a relationship()
placed on Employee
which links to Company
does not work
against the Manager
and Engineer
classes, when exercised at the
instance level. Instead, a distinct
relationship()
must be applied to each class. In order to achieve
bi-directional behavior in terms of three separate relationships which
serve as the opposite of Company.employees
, the
relationship.back_populates
parameter is used between
each of the relationships:
from sqlalchemy.ext.declarative import ConcreteBase
class Company(Base):
__tablename__ = "company"
id = Column(Integer, primary_key=True)
name = Column(String(50))
employees = relationship("Employee", back_populates="company")
class Employee(ConcreteBase, Base):
__tablename__ = "employee"
id = Column(Integer, primary_key=True)
name = Column(String(50))
company_id = Column(ForeignKey("company.id"))
company = relationship("Company", back_populates="employees")
__mapper_args__ = {
"polymorphic_identity": "employee",
"concrete": True,
}
class Manager(Employee):
__tablename__ = "manager"
id = Column(Integer, primary_key=True)
name = Column(String(50))
manager_data = Column(String(40))
company_id = Column(ForeignKey("company.id"))
company = relationship("Company", back_populates="employees")
__mapper_args__ = {
"polymorphic_identity": "manager",
"concrete": True,
}
class Engineer(Employee):
__tablename__ = "engineer"
id = Column(Integer, primary_key=True)
name = Column(String(50))
engineer_info = Column(String(40))
company_id = Column(ForeignKey("company.id"))
company = relationship("Company", back_populates="employees")
__mapper_args__ = {
"polymorphic_identity": "engineer",
"concrete": True,
}
The above limitation is related to the current implementation, including that concrete inheriting classes do not share any of the attributes of the superclass and therefore need distinct relationships to be set up.
Loading Concrete Inheritance Mappings¶
The options for loading with concrete inheritance are limited; generally,
if polymorphic loading is configured on the mapper using one of the
declarative concrete mixins, it can’t be modified at query time
in current SQLAlchemy versions. Normally, the with_polymorphic()
function would be able to override the style of loading used by concrete,
however due to current limitations this is not yet supported.