Django/venv/lib/python3.7/site-packages/django/db/models/constraints.py

from django.db.models.query_utils import Q
from django.db.models.sql.query import Query

__all__ = ['CheckConstraint', 'UniqueConstraint']


class BaseConstraint:
    def __init__(self, name):
        self.name = name

    def constraint_sql(self, model, schema_editor):
        raise NotImplementedError('This method must be implemented by a subclass.')

    def create_sql(self, model, schema_editor):
        raise NotImplementedError('This method must be implemented by a subclass.')

    def remove_sql(self, model, schema_editor):
        raise NotImplementedError('This method must be implemented by a subclass.')

    def deconstruct(self):
        path = '%s.%s' % (self.__class__.__module__, self.__class__.__name__)
        path = path.replace('django.db.models.constraints', 'django.db.models')
        return (path, (), {'name': self.name})

    def clone(self):
        _, args, kwargs = self.deconstruct()
        return self.__class__(*args, **kwargs)


class CheckConstraint(BaseConstraint):
    def __init__(self, *, check, name):
        self.check = check
        super().__init__(name)

    def _get_check_sql(self, model, schema_editor):
        query = Query(model=model)
        where = query.build_where(self.check)
        compiler = query.get_compiler(connection=schema_editor.connection)
        sql, params = where.as_sql(compiler, schema_editor.connection)
        return sql % tuple(schema_editor.quote_value(p) for p in params)

    def constraint_sql(self, model, schema_editor):
        check = self._get_check_sql(model, schema_editor)
        return schema_editor._check_sql(self.name, check)

    def create_sql(self, model, schema_editor):
        check = self._get_check_sql(model, schema_editor)
        return schema_editor._create_check_sql(model, self.name, check)

    def remove_sql(self, model, schema_editor):
        return schema_editor._delete_check_sql(model, self.name)

    def __repr__(self):
        return "<%s: check='%s' name=%r>" % (self.__class__.__name__, self.check, self.name)

    def __eq__(self, other):
        return (
            isinstance(other, CheckConstraint) and
            self.name == other.name and
            self.check == other.check
        )

    def deconstruct(self):
        path, args, kwargs = super().deconstruct()
        kwargs['check'] = self.check
        return path, args, kwargs


class UniqueConstraint(BaseConstraint):
    def __init__(self, *, fields, name, condition=None):
        if not fields:
            raise ValueError('At least one field is required to define a unique constraint.')
        if not isinstance(condition, (type(None), Q)):
            raise ValueError('UniqueConstraint.condition must be a Q instance.')
        self.fields = tuple(fields)
        self.condition = condition
        super().__init__(name)

    def _get_condition_sql(self, model, schema_editor):
        if self.condition is None:
            return None
        query = Query(model=model)
        where = query.build_where(self.condition)
        compiler = query.get_compiler(connection=schema_editor.connection)
        sql, params = where.as_sql(compiler, schema_editor.connection)
        return sql % tuple(schema_editor.quote_value(p) for p in params)

    def constraint_sql(self, model, schema_editor):
        fields = [model._meta.get_field(field_name).column for field_name in self.fields]
        condition = self._get_condition_sql(model, schema_editor)
        return schema_editor._unique_sql(model, fields, self.name, condition=condition)

    def create_sql(self, model, schema_editor):
        fields = [model._meta.get_field(field_name).column for field_name in self.fields]
        condition = self._get_condition_sql(model, schema_editor)
        return schema_editor._create_unique_sql(model, fields, self.name, condition=condition)

    def remove_sql(self, model, schema_editor):
        condition = self._get_condition_sql(model, schema_editor)
        return schema_editor._delete_unique_sql(model, self.name, condition=condition)

    def __repr__(self):
        return '<%s: fields=%r name=%r%s>' % (
            self.__class__.__name__, self.fields, self.name,
            '' if self.condition is None else ' condition=%s' % self.condition,
        )

    def __eq__(self, other):
        return (
            isinstance(other, UniqueConstraint) and
            self.name == other.name and
            self.fields == other.fields and
            self.condition == other.condition
        )

    def deconstruct(self):
        path, args, kwargs = super().deconstruct()
        kwargs['fields'] = self.fields
        if self.condition:
            kwargs['condition'] = self.condition
        return path, args, kwargs