Fixed #20581 -- Added support for deferrable unique constraints.

7 files changed, 99 insertions, 14 deletions

diff --git a/django/db/backends/base/features.py b/django/db/backends/base/features.py
index c2b92ecdee..a8f55f966c 100644
--- a/django/db/backends/base/features.py
+++ b/django/db/backends/base/features.py
@@ -20,6 +20,8 @@ class BaseDatabaseFeatures:
     # Does the backend allow inserting duplicate rows when a unique_together
     # constraint exists and some fields are nullable but not all of them?
     supports_partially_nullable_unique_constraints = True
+    # Does the backend support initially deferrable unique constraints?
+    supports_deferrable_unique_constraints = False
 
     can_use_chunked_reads = True
     can_return_columns_from_insert = False
diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index 104f569a07..2b2ad9cdb4 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -5,7 +5,7 @@ from django.db.backends.ddl_references import (
     Columns, ForeignKeyName, IndexName, Statement, Table,
 )
 from django.db.backends.utils import names_digest, split_identifier
-from django.db.models import Index
+from django.db.models import Deferrable, Index
 from django.db.transaction import TransactionManagementError, atomic
 from django.utils import timezone
 
@@ -65,7 +65,7 @@ class BaseDatabaseSchemaEditor:
     sql_rename_column = "ALTER TABLE %(table)s RENAME COLUMN %(old_column)s TO %(new_column)s"
     sql_update_with_default = "UPDATE %(table)s SET %(column)s = %(default)s WHERE %(column)s IS NULL"
 
-    sql_unique_constraint = "UNIQUE (%(columns)s)"
+    sql_unique_constraint = "UNIQUE (%(columns)s)%(deferrable)s"
     sql_check_constraint = "CHECK (%(check)s)"
     sql_delete_constraint = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"
     sql_constraint = "CONSTRAINT %(name)s %(constraint)s"
@@ -73,7 +73,7 @@ class BaseDatabaseSchemaEditor:
     sql_create_check = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s CHECK (%(check)s)"
     sql_delete_check = sql_delete_constraint
 
-    sql_create_unique = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s UNIQUE (%(columns)s)"
+    sql_create_unique = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s UNIQUE (%(columns)s)%(deferrable)s"
     sql_delete_unique = sql_delete_constraint
 
     sql_create_fk = (
@@ -1075,7 +1075,20 @@ class BaseDatabaseSchemaEditor:
     def _delete_fk_sql(self, model, name):
         return self._delete_constraint_sql(self.sql_delete_fk, model, name)
 
-    def _unique_sql(self, model, fields, name, condition=None):
+    def _deferrable_constraint_sql(self, deferrable):
+        if deferrable is None:
+            return ''
+        if deferrable == Deferrable.DEFERRED:
+            return ' DEFERRABLE INITIALLY DEFERRED'
+        if deferrable == Deferrable.IMMEDIATE:
+            return ' DEFERRABLE INITIALLY IMMEDIATE'
+
+    def _unique_sql(self, model, fields, name, condition=None, deferrable=None):
+        if (
+            deferrable and
+            not self.connection.features.supports_deferrable_unique_constraints
+        ):
+            return None
         if condition:
             # Databases support conditional unique constraints via a unique
             # index.
@@ -1085,13 +1098,20 @@ class BaseDatabaseSchemaEditor:
             return None
         constraint = self.sql_unique_constraint % {
             'columns': ', '.join(map(self.quote_name, fields)),
+            'deferrable': self._deferrable_constraint_sql(deferrable),
         }
         return self.sql_constraint % {
             'name': self.quote_name(name),
             'constraint': constraint,
         }
 
-    def _create_unique_sql(self, model, columns, name=None, condition=None):
+    def _create_unique_sql(self, model, columns, name=None, condition=None, deferrable=None):
+        if (
+            deferrable and
+            not self.connection.features.supports_deferrable_unique_constraints
+        ):
+            return None
+
         def create_unique_name(*args, **kwargs):
             return self.quote_name(self._create_index_name(*args, **kwargs))
 
@@ -1113,9 +1133,15 @@ class BaseDatabaseSchemaEditor:
             name=name,
             columns=columns,
             condition=self._index_condition_sql(condition),
+            deferrable=self._deferrable_constraint_sql(deferrable),
         )
 
-    def _delete_unique_sql(self, model, name, condition=None):
+    def _delete_unique_sql(self, model, name, condition=None, deferrable=None):
+        if (
+            deferrable and
+            not self.connection.features.supports_deferrable_unique_constraints
+        ):
+            return None
         if condition:
             return (
                 self._delete_constraint_sql(self.sql_delete_index, model, name)
diff --git a/django/db/backends/oracle/base.py b/django/db/backends/oracle/base.py
index 8285aacdc1..e9ec2bac51 100644
--- a/django/db/backends/oracle/base.py
+++ b/django/db/backends/oracle/base.py
@@ -71,9 +71,17 @@ def wrap_oracle_errors():
         #  message = 'ORA-02091: transaction rolled back
         #            'ORA-02291: integrity constraint (TEST_DJANGOTEST.SYS
         #               _C00102056) violated - parent key not found'
+        #            or:
+        #            'ORA-00001: unique constraint (DJANGOTEST.DEFERRABLE_
+        #               PINK_CONSTRAINT) violated
         # Convert that case to Django's IntegrityError exception.
         x = e.args[0]
-        if hasattr(x, 'code') and hasattr(x, 'message') and x.code == 2091 and 'ORA-02291' in x.message:
+        if (
+            hasattr(x, 'code') and
+            hasattr(x, 'message') and
+            x.code == 2091 and
+            ('ORA-02291' in x.message or 'ORA-00001' in x.message)
+        ):
             raise IntegrityError(*tuple(e.args))
         raise
 
diff --git a/django/db/backends/oracle/features.py b/django/db/backends/oracle/features.py
index 24b281281d..3782874512 100644
--- a/django/db/backends/oracle/features.py
+++ b/django/db/backends/oracle/features.py
@@ -17,6 +17,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_native_duration_field = True
     can_defer_constraint_checks = True
     supports_partially_nullable_unique_constraints = False
+    supports_deferrable_unique_constraints = True
     truncates_names = True
     supports_tablespaces = True
     supports_sequence_reset = False
diff --git a/django/db/backends/postgresql/features.py b/django/db/backends/postgresql/features.py
index 533c8dc282..3b4199fa78 100644
--- a/django/db/backends/postgresql/features.py
+++ b/django/db/backends/postgresql/features.py
@@ -56,6 +56,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_aggregate_filter_clause = True
     supported_explain_formats = {'JSON', 'TEXT', 'XML', 'YAML'}
     validates_explain_options = False  # A query will error on invalid options.
+    supports_deferrable_unique_constraints = True
 
     @cached_property
     def is_postgresql_9_6(self):
diff --git a/django/db/models/base.py b/django/db/models/base.py
index 0f8a8c50f3..6c9e9d3707 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -1904,6 +1904,25 @@ class Model(metaclass=ModelBase):
                         id='models.W036',
                     )
                 )
+            if not (
+                connection.features.supports_deferrable_unique_constraints or
+                'supports_deferrable_unique_constraints' in cls._meta.required_db_features
+            ) and any(
+                isinstance(constraint, UniqueConstraint) and constraint.deferrable is not None
+                for constraint in cls._meta.constraints
+            ):
+                errors.append(
+                    checks.Warning(
+                        '%s does not support deferrable unique constraints.'
+                        % connection.display_name,
+                        hint=(
+                            "A constraint won't be created. Silence this "
+                            "warning if you don't care about it."
+                        ),
+                        obj=cls,
+                        id='models.W038',
+                    )
+                )
         return errors
 
 
diff --git a/django/db/models/constraints.py b/django/db/models/constraints.py
index 98912a6467..64bd60484e 100644
--- a/django/db/models/constraints.py
+++ b/django/db/models/constraints.py
@@ -1,7 +1,9 @@
+from enum import Enum
+
 from django.db.models.query_utils import Q
 from django.db.models.sql.query import Query
 
-__all__ = ['CheckConstraint', 'UniqueConstraint']
+__all__ = ['CheckConstraint', 'Deferrable', 'UniqueConstraint']
 
 
 class BaseConstraint:
@@ -69,14 +71,28 @@ class CheckConstraint(BaseConstraint):
         return path, args, kwargs
 
 
+class Deferrable(Enum):
+    DEFERRED = 'deferred'
+    IMMEDIATE = 'immediate'
+
+
 class UniqueConstraint(BaseConstraint):
-    def __init__(self, *, fields, name, condition=None):
+    def __init__(self, *, fields, name, condition=None, deferrable=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.')
+        if condition and deferrable:
+            raise ValueError(
+                'UniqueConstraint with conditions cannot be deferred.'
+            )
+        if not isinstance(deferrable, (type(None), Deferrable)):
+            raise ValueError(
+                'UniqueConstraint.deferrable must be a Deferrable instance.'
+            )
         self.fields = tuple(fields)
         self.condition = condition
+        self.deferrable = deferrable
         super().__init__(name)
 
     def _get_condition_sql(self, model, schema_editor):
@@ -91,21 +107,30 @@ class UniqueConstraint(BaseConstraint):
     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)
+        return schema_editor._unique_sql(
+            model, fields, self.name, condition=condition,
+            deferrable=self.deferrable,
+        )
 
     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)
+        return schema_editor._create_unique_sql(
+            model, fields, self.name, condition=condition,
+            deferrable=self.deferrable,
+        )
 
     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)
+        return schema_editor._delete_unique_sql(
+            model, self.name, condition=condition, deferrable=self.deferrable,
+        )
 
     def __repr__(self):
-        return '<%s: fields=%r name=%r%s>' % (
+        return '<%s: fields=%r name=%r%s%s>' % (
             self.__class__.__name__, self.fields, self.name,
             '' if self.condition is None else ' condition=%s' % self.condition,
+            '' if self.deferrable is None else ' deferrable=%s' % self.deferrable,
         )
 
     def __eq__(self, other):
@@ -113,7 +138,8 @@ class UniqueConstraint(BaseConstraint):
             return (
                 self.name == other.name and
                 self.fields == other.fields and
-                self.condition == other.condition
+                self.condition == other.condition and
+                self.deferrable == other.deferrable
             )
         return super().__eq__(other)
 
@@ -122,4 +148,6 @@ class UniqueConstraint(BaseConstraint):
         kwargs['fields'] = self.fields
         if self.condition:
             kwargs['condition'] = self.condition
+        if self.deferrable:
+            kwargs['deferrable'] = self.deferrable
         return path, args, kwargs