ormar/docs/models/inheritance.md

# 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.

```python
# 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.

```python
# 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.

```python
# 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:

```python
# 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:

```python
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.

```python
# 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.

```python
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`.