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 it's own table with columns from 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.

# 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()

!!!note 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, with Category class fields combined with all Mixins fields.

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

!!!warning You cannot declare a field in a Model that is already defined in one of the Mixins you inherit from.

So in example above `Category` cannot declare it's own `created_date` as this filed will be inherited from `DateFieldsMixins`.

If you try to the `ModelDefinitionError` will be raised.

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 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 it - otherwise an error will be raised.

# 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.

!!!warning You cannot declare a field in a Model that is already defined in one of the Mixins you inherit from.

So in example above `Category` cannot declare it's own `created_date` as this filed will be inherited from `DateFieldsMixins`.

If you try to the `ModelDefinitionError` will be raised.

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.

# 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 one
        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.

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

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