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Inheritance

Out of various types of ORM models inheritance ormar currently supports two of them:

  • Mixins
  • Concrete table inheritance (with parents set to abstract=True)

Types of inheritance

The short summary of different types of inheritance is:

  • Mixins [SUPPORTED] - don't subclass ormar.Model, just define fields that are later used on different models (like created_date and updated_date on each model), only actual models create tables, but those fields from mixins are added
  • Concrete table inheritance [SUPPORTED] - means that parent is marked as abstract and each child has its own table with columns from a parent and own child columns, kind of similar to Mixins but parent also is a Model
  • Single table inheritance [NOT SUPPORTED] - means that only one table is created with fields that are combination/sum of the parent and all children models but child models use only subset of column in db (all parent and own ones, skipping the other children ones)
  • Multi/ Joined table inheritance [NOT SUPPORTED] - means that part of the columns is saved on parent model and part is saved on child model that are connected to each other by kind of one to one relation and under the hood you operate on two models at once
  • Proxy models [NOT SUPPORTED] - means that only parent has an actual table, children just add methods, modify settings etc.

Mixins

To use Mixins just define a class that is not inheriting from an ormar.Model but is defining ormar.Fields as class variables.

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# a mixin defines the fields but is a normal python class 
class AuditMixin:
    created_by: str = ormar.String(max_length=100)
    updated_by: str = ormar.String(max_length=100, default="Sam")


class DateFieldsMixins:
    created_date: datetime.datetime = ormar.DateTime(default=datetime.datetime.now)
    updated_date: datetime.datetime = ormar.DateTime(default=datetime.datetime.now)


# a models can inherit from one or more mixins
class Category(ormar.Model, DateFieldsMixins, AuditMixin):
    class Meta(ormar.ModelMeta):
        tablename = "categories"
        metadata = metadata
        database = db

    id: int = ormar.Integer(primary_key=True)
    name: str = ormar.String(max_length=50, unique=True, index=True)
    code: int = ormar.Integer()

Tip

Note that Mixins are not models, so you still need to inherit from ormar.Model as well as define Meta class in the final model.

A Category class above will have four additional fields: created_date, updated_date, created_by and updated_by.

There will be only one table created for model Category (categories), with Category class fields combined with all Mixins fields.

Note that Mixin in class name is optional but is a good python practice.

Concrete table inheritance

In concept concrete table inheritance is very similar to Mixins, but uses actual ormar.Models as base classes.

Warning

Note that base classes have abstract=True set in Meta class, if you try to inherit from non abstract marked class ModelDefinitionError will be raised.

Since this abstract Model will never be initialized you can skip metadata and database in it's Meta definition.

But if you provide it - it will be inherited, that way you do not have to provide metadata and databases in the final/concrete class

Note that you can always overwrite it in child/concrete class if you need to.

More over at least one of the classes in inheritance chain have to provide both database and metadata - otherwise an error will be raised.

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# note that base classes have abstract=True
# since this model will never be initialized you can skip metadata and database
class AuditModel(ormar.Model):
    class Meta:
        abstract = True

    created_by: str = ormar.String(max_length=100)
    updated_by: str = ormar.String(max_length=100, default="Sam")


# but if you provide it it will be inherited - DRY (Don't Repeat Yourself) in action
class DateFieldsModel(ormar.Model):
    class Meta:
        abstract = True
        metadata = metadata
        database = db

    created_date: datetime.datetime = ormar.DateTime(default=datetime.datetime.now)
    updated_date: datetime.datetime = ormar.DateTime(default=datetime.datetime.now)


# that way you do not have to provide metadata and databases in concrete class
class Category(DateFieldsModel, AuditModel):
    class Meta(ormar.ModelMeta):
        tablename = "categories"

    id: int = ormar.Integer(primary_key=True)
    name: str = ormar.String(max_length=50, unique=True, index=True)
    code: int = ormar.Integer()

The list of inherited options/settings is as follows: metadata, database and constraints.

Also methods decorated with @property_field decorator will be inherited/recognized.

Of course apart from that all fields from base classes are combined and created in the concrete table of the final Model.

Tip

Note how you don't have to provide abstarct=False in the final class - it's the default setting that is not inherited.

Redefining fields in subclasses

Note that you can redefine previously created fields like in normal python class inheritance.

Whenever you define a field with same name and new definition it will completely replace the previously defined one.

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# base class
class DateFieldsModel(ormar.Model):
    class Meta:
        abstract = True
        metadata = metadata
        database = db
        # note that UniqueColumns need sqlalchemy db columns names not the ormar ones
        constraints = [ormar.UniqueColumns("creation_date", "modification_date")]

    created_date: datetime.datetime = ormar.DateTime(
        default=datetime.datetime.now, name="creation_date"
    )
    updated_date: datetime.datetime = ormar.DateTime(
        default=datetime.datetime.now, name="modification_date"
    )


class RedefinedField(DateFieldsModel):
    class Meta(ormar.ModelMeta):
        tablename = "redefines"
        metadata = metadata
        database = db

    id: int = ormar.Integer(primary_key=True)
    # here the created_date is replaced by the String field
    created_date: str = ormar.String(max_length=200, name="creation_date")


# you can verify that the final field is correctly declared and created
changed_field = RedefinedField.Meta.model_fields["created_date"]
assert changed_field.default is None
assert changed_field.alias == "creation_date"
assert any(x.name == "creation_date" for x in RedefinedField.Meta.table.columns)
assert isinstance(
    RedefinedField.Meta.table.columns["creation_date"].type,
    sqlalchemy.sql.sqltypes.String,
)

Warning

If you declare UniqueColumns constraint with column names, the final model has to have a column with the same name declared. Otherwise, the ModelDefinitionError will be raised.

So in example above if you do not provide name for created_date in RedefinedField model ormar will complain.

created_date: str = ormar.String(max_length=200) # exception

created_date: str = ormar.String(max_length=200, name="creation_date2") # exception

Relations in inheritance

You can declare relations in every step of inheritance, so both in parent and child classes.

When you define a relation on a child model level it's either overwriting the relation defined in parent model (if the same field name is used), or is accessible only to this child if you define a new relation.

When inheriting relations, you always need to be aware of related_name parameter, that has to be unique across a related model, when you define multiple child classes that inherit the same relation.

If you do not provide related_name parameter ormar calculates it for you. This works with inheritance as all child models have to have different class names, which are used to calculate the default related_name (class.name.lower()+'s').

But, if you provide a related_name this name cannot be reused in all child models as they would overwrite each other on the related model side.

Therefore, you have two options:

  • redefine relation field in child models and manually provide different related_name parameters
  • let this for ormar to handle -> auto adjusted related_name are: original related_name + "_" + child model table name

That might sound complicated but let's look at the following example:

ForeignKey relations

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# normal model used in relation
class Person(ormar.Model):
    class Meta:
        metadata = metadata
        database = db

    id: int = ormar.Integer(primary_key=True)
    name: str = ormar.String(max_length=100)


# parent model - needs to be abstract
class Car(ormar.Model):
    class Meta:
        abstract = True
        metadata = metadata
        database = db

    id: int = ormar.Integer(primary_key=True)
    name: str = ormar.String(max_length=50)
    owner: Person = ormar.ForeignKey(Person)
    # note that we refer to the Person model again so we **have to** provide related_name
    co_owner: Person = ormar.ForeignKey(Person, related_name="coowned")
    created_date: datetime.datetime = ormar.DateTime(default=datetime.datetime.now)


class Truck(Car):
    class Meta:
        pass

    max_capacity: int = ormar.Integer()


class Bus(Car):
    class Meta:
        # default naming is name.lower()+'s' so it's ugly for buss ;)
        tablename = "buses"

    max_persons: int = ormar.Integer()

Now when you will inspect the fields on Person model you will get:

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Person.Meta.model_fields
"""
{'id': <class 'ormar.fields.model_fields.Integer'>, 
'name': <class 'ormar.fields.model_fields.String'>, 
'trucks': <class 'ormar.fields.foreign_key.ForeignKey'>, 
'coowned_trucks': <class 'ormar.fields.foreign_key.ForeignKey'>, 
'buss': <class 'ormar.fields.foreign_key.ForeignKey'>, 
'coowned_buses': <class 'ormar.fields.foreign_key.ForeignKey'>}
"""

Note how you have trucks and buss fields that leads to Truck and Bus class that this Person owns. There were no related_name parameter so default names were used.

At the same time the co-owned cars need to be referenced by coowned_trucks and coowned_buses. Ormar appended _trucks and _buses suffixes taken from child model table names.

Seems fine, but the default name for owned trucks is ok (trucks) but the buss is ugly, so how can we change it?

The solution is pretty simple - just redefine the field in Bus class and provide different related_name parameter.

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# rest of the above example remains the same
class Bus(Car):
    class Meta:
        tablename = "buses"

    # new field that changes the related_name
    owner: Person = ormar.ForeignKey(Person, related_name="buses")
    max_persons: int = ormar.Integer()

Now the columns looks much better.

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Person.Meta.model_fields
"""
{'id': <class 'ormar.fields.model_fields.Integer'>, 
'name': <class 'ormar.fields.model_fields.String'>, 
'trucks': <class 'ormar.fields.foreign_key.ForeignKey'>, 
'coowned_trucks': <class 'ormar.fields.foreign_key.ForeignKey'>, 
'buses': <class 'ormar.fields.foreign_key.ForeignKey'>, 
'coowned_buses': <class 'ormar.fields.foreign_key.ForeignKey'>}
"""

Note

You could also provide related_name for the owner field, that way the proper suffixes would be added.

owner: Person = ormar.ForeignKey(Person, related_name="owned")

and model fields for Person owned cars would become owned_trucks and owned_buses.

ManyToMany relations

Similarly, you can inherit from Models that have ManyToMany relations declared but there is one, but substantial difference - the Through model.

Since in the future the Through model will be able to hold additional fields and now it links only two Tables (from and to ones), each child that inherits the m2m relation field has to have separate Through model.

Of course, you can overwrite the relation in each Child model, but that requires additional code and undermines the point of the whole inheritance. Ormar will handle this for you if you agree with default naming convention, which you can always manually overwrite in children if needed.

Again, let's look at the example to easier grasp the concepts.

We will modify the previous example described above to use m2m relation for co_owners.

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# person remain the same as above
class Person(ormar.Model):
    class Meta:
        metadata = metadata
        database = db

    id: int = ormar.Integer(primary_key=True)
    name: str = ormar.String(max_length=100)

# new through model between Person and Car2
class PersonsCar(ormar.Model):
    class Meta:
        tablename = "cars_x_persons"
        metadata = metadata
        database = db

# note how co_owners is now ManyToMany relation
class Car2(ormar.Model):
    class Meta:
        # parent class needs to be marked abstract
        abstract = True
        metadata = metadata
        database = db

    id: int = ormar.Integer(primary_key=True)
    name: str = ormar.String(max_length=50)
    # note the related_name - needs to be unique across Person
    # model, regardless of how many different models leads to Person
    owner: Person = ormar.ForeignKey(Person, related_name="owned")
    co_owners: List[Person] = ormar.ManyToMany(
        Person, through=PersonsCar, related_name="coowned"
    )
    created_date: datetime.datetime = ormar.DateTime(default=datetime.datetime.now)


# child models define only additional Fields
class Truck2(Car2):
    class Meta:
        # note how you don't have to provide inherited Meta params
        tablename = "trucks2"

    max_capacity: int = ormar.Integer()


class Bus2(Car2):
    class Meta:
        tablename = "buses2"

    max_persons: int = ormar.Integer()

Ormar automatically modifies related_name of the fields to include the table name of the children models. The dafault name is original related_name + '_' + child table name.

That way for class Truck2 the relation defined in owner: Person = ormar.ForeignKey(Person, related_name="owned") becomes owned_trucks2

You can verify the names by inspecting the list of fields present on Person model.

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Person.Meta.model_fields
{
# note how all relation fields need to be unique on Person
# regardless if autogenerated or manually overwritten
'id': <class 'ormar.fields.model_fields.Integer'>, 
'name': <class 'ormar.fields.model_fields.String'>, 
# note that we expanded on previous example so all 'old' fields are here
'trucks': <class 'ormar.fields.foreign_key.ForeignKey'>, 
'coowned_trucks': <class 'ormar.fields.foreign_key.ForeignKey'>, 
'buses': <class 'ormar.fields.foreign_key.ForeignKey'>, 
'coowned_buses': <class 'ormar.fields.foreign_key.ForeignKey'>, 
# newly defined related fields
'owned_trucks2': <class 'ormar.fields.foreign_key.ForeignKey'>, 
'coowned_trucks2': <class 'abc.ManyToMany'>, 
'owned_buses2': <class 'ormar.fields.foreign_key.ForeignKey'>, 
'coowned_buses2': <class 'abc.ManyToMany'>
}

But that's not all. It's kind of internal to ormar but affects the data structure in the database, so let's examine the through models for both Bus2 and Truck2 models.

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Bus2.Meta.model_fields['co_owners'].through
<class 'abc.PersonsCarBus2'>
Bus2.Meta.model_fields['co_owners'].through.Meta.tablename
'cars_x_persons_buses2'

Truck2.Meta.model_fields['co_owners'].through
<class 'abc.PersonsCarTruck2'>
Truck2.Meta.model_fields['co_owners'].through.Meta.tablename
'cars_x_persons_trucks2'

As you can see above ormar cloned the Through model for each of the Child classes and added Child class name at the end, while changing the table names of the cloned fields the name of the table from the child is used.

Note that original model is not only not used, the table for this model is removed from metadata:

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Bus2.Meta.metadata.tables.keys()
dict_keys(['test_date_models', 'categories', 'subjects', 'persons', 'trucks', 'buses', 
           'cars_x_persons_trucks2', 'trucks2', 'cars_x_persons_buses2', 'buses2'])

So be aware that if you introduce inheritance along the way and convert a model into abstract parent model you may lose your data on through table if not careful.

Note

Note that original table name and model name of the Through model is never used. Only the cloned models tables are created and used.

Warning

Note that each subclass of the Model that has ManyToMany relation defined generates a new Through model, meaning also new database table.

That means that each time you define a Child model you need to either manually create the table in the database, or run a migration (with alembic).

exclude_parent_fields

Ormar allows you to skip certain fields in inherited model that are coming from a parent model.

Note

Note that the same behaviour can be achieved by splitting the model into more abstract models and mixins - which is a preferred way in normal circumstances.

To skip certain fields from a child model, list all fields that you want to skip in model.Meta.exclude_parent_fields parameter like follows:

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metadata = sa.MetaData()
db = databases.Database(DATABASE_URL)


class AuditModel(ormar.Model):
    class Meta:
        abstract = True

    created_by: str = ormar.String(max_length=100)
    updated_by: str = ormar.String(max_length=100, default="Sam")


class DateFieldsModel(ormar.Model):
    class Meta(ormar.ModelMeta):
        abstract = True
        metadata = metadata
        database = db

    created_date: datetime.datetime = ormar.DateTime(
        default=datetime.datetime.now, name="creation_date"
    )
    updated_date: datetime.datetime = ormar.DateTime(
        default=datetime.datetime.now, name="modification_date"
    )


class Category(DateFieldsModel, AuditModel):
    class Meta(ormar.ModelMeta):
        tablename = "categories"
        # set fields that should be skipped
        exclude_parent_fields = ["updated_by", "updated_date"]

    id: int = ormar.Integer(primary_key=True)
    name: str = ormar.String(max_length=50, unique=True, index=True)
    code: int = ormar.Integer()

# Note that now the update fields in Category are gone in all places -> ormar fields, pydantic fields and sqlachemy table columns
# so full list of available fileds in Category is: ["created_by", "created_date", "id", "name", "code"]

Note how you simply need to provide field names and it will exclude the parent field regardless of from which parent model the field is coming from.

Note

Note that if you want to overwrite a field in child model you do not have to exclude it, simpy overwrite the field declaration in child model with same field name.

Warning

Note that this kind of behavior can confuse mypy and static type checkers, yet accessing the non existing fields will fail at runtime. That's why splitting the base classes is preferred.

The same effect can be achieved by splitting base classes like:

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metadata = sa.MetaData()
db = databases.Database(DATABASE_URL)


class AuditCreateModel(ormar.Model):
    class Meta:
        abstract = True

    created_by: str = ormar.String(max_length=100)


class AuditUpdateModel(ormar.Model):
    class Meta:
        abstract = True

    updated_by: str = ormar.String(max_length=100, default="Sam")

class CreateDateFieldsModel(ormar.Model):
    class Meta(ormar.ModelMeta):
        abstract = True
        metadata = metadata
        database = db

    created_date: datetime.datetime = ormar.DateTime(
        default=datetime.datetime.now, name="creation_date"
    )

class UpdateDateFieldsModel(ormar.Model):
    class Meta(ormar.ModelMeta):
        abstract = True
        metadata = metadata
        database = db

    updated_date: datetime.datetime = ormar.DateTime(
        default=datetime.datetime.now, name="modification_date"
    )


class Category(CreateDateFieldsModel, AuditCreateModel):
    class Meta(ormar.ModelMeta):
        tablename = "categories"

    id: int = ormar.Integer(primary_key=True)
    name: str = ormar.String(max_length=50, unique=True, index=True)
    code: int = ormar.Integer()

That way you can inherit from both create and update classes if needed, and only one of them otherwise.