Fixed #29824 -- Added support for database exclusion constraints on PostgreSQL.

Thanks to Nick Pope and Mariusz Felisiak for review.

Co-Authored-By: Mariusz Felisiak <felisiak.mariusz@gmail.com>

django/contrib/postgres/constraints.py

@@ -0,0 +1,106 @@
+from django.db.backends.ddl_references import Statement, Table
+from django.db.models import F, Q
+from django.db.models.constraints import BaseConstraint
+from django.db.models.sql import Query
+
+__all__ = ['ExclusionConstraint']
+
+
+class ExclusionConstraint(BaseConstraint):
+    template = 'CONSTRAINT %(name)s EXCLUDE USING %(index_type)s (%(expressions)s)%(where)s'
+
+    def __init__(self, *, name, expressions, index_type=None, condition=None):
+        if index_type and index_type.lower() not in {'gist', 'spgist'}:
+            raise ValueError(
+                'Exclusion constraints only support GiST or SP-GiST indexes.'
+            )
+        if not expressions:
+            raise ValueError(
+                'At least one expression is required to define an exclusion '
+                'constraint.'
+            )
+        if not all(
+            isinstance(expr, (list, tuple)) and len(expr) == 2
+            for expr in expressions
+        ):
+            raise ValueError('The expressions must be a list of 2-tuples.')
+        if not isinstance(condition, (type(None), Q)):
+            raise ValueError(
+                'ExclusionConstraint.condition must be a Q instance.'
+            )
+        self.expressions = expressions
+        self.index_type = index_type or 'GIST'
+        self.condition = condition
+        super().__init__(name=name)
+
+    def _get_expression_sql(self, compiler, connection, query):
+        expressions = []
+        for expression, operator in self.expressions:
+            if isinstance(expression, str):
+                expression = F(expression)
+            if isinstance(expression, F):
+                expression = expression.resolve_expression(query=query, simple_col=True)
+            else:
+                expression = expression.resolve_expression(query=query)
+            sql, params = expression.as_sql(compiler, connection)
+            expressions.append('%s WITH %s' % (sql % params, operator))
+        return expressions
+
+    def _get_condition_sql(self, compiler, schema_editor, query):
+        if self.condition is None:
+            return None
+        where = query.build_where(self.condition)
+        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):
+        query = Query(model)
+        compiler = query.get_compiler(connection=schema_editor.connection)
+        expressions = self._get_expression_sql(compiler, schema_editor.connection, query)
+        condition = self._get_condition_sql(compiler, schema_editor, query)
+        return self.template % {
+            'name': schema_editor.quote_name(self.name),
+            'index_type': self.index_type,
+            'expressions': ', '.join(expressions),
+            'where': ' WHERE (%s)' % condition if condition else '',
+        }
+
+    def create_sql(self, model, schema_editor):
+        return Statement(
+            'ALTER TABLE %(table)s ADD %(constraint)s',
+            table=Table(model._meta.db_table, schema_editor.quote_name),
+            constraint=self.constraint_sql(model, schema_editor),
+        )
+
+    def remove_sql(self, model, schema_editor):
+        return schema_editor._delete_constraint_sql(
+            schema_editor.sql_delete_check,
+            model,
+            schema_editor.quote_name(self.name),
+        )
+
+    def deconstruct(self):
+        path, args, kwargs = super().deconstruct()
+        kwargs['expressions'] = self.expressions
+        if self.condition is not None:
+            kwargs['condition'] = self.condition
+        if self.index_type.lower() != 'gist':
+            kwargs['index_type'] = self.index_type
+        return path, args, kwargs
+
+    def __eq__(self, other):
+        return (
+            isinstance(other, self.__class__) and
+            self.name == other.name and
+            self.index_type == other.index_type and
+            self.expressions == other.expressions and
+            self.condition == other.condition
+        )
+
+    def __repr__(self):
+        return '<%s: index_type=%s, expressions=%s%s>' % (
+            self.__class__.__qualname__,
+            self.index_type,
+            self.expressions,
+            '' if self.condition is None else ', condition=%s' % self.condition,
+        )