Fixed #30062 -- Added support for unique conditional constraints.

4 files changed, 77 insertions, 20 deletions

diff --git a/django/db/backends/base/schema.py b/django/db/backends/base/schema.py
index 578bfb244c..113d1b7f67 100644
--- a/django/db/backends/base/schema.py
+++ b/django/db/backends/base/schema.py
@@ -80,6 +80,7 @@ class BaseDatabaseSchemaEditor:
     sql_delete_fk = sql_delete_constraint
 
     sql_create_index = "CREATE INDEX %(name)s ON %(table)s (%(columns)s)%(extra)s%(condition)s"
+    sql_create_unique_index = "CREATE UNIQUE INDEX %(name)s ON %(table)s (%(columns)s)%(condition)s"
     sql_delete_index = "DROP INDEX %(name)s"
 
     sql_create_pk = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s PRIMARY KEY (%(columns)s)"
@@ -296,7 +297,7 @@ class BaseDatabaseSchemaEditor:
         # Make the table
         sql = self.sql_create_table % {
             "table": self.quote_name(model._meta.db_table),
-            "definition": ", ".join((*column_sqls, *constraints)),
+            "definition": ", ".join(constraint for constraint in (*column_sqls, *constraints) if constraint),
         }
         if model._meta.db_tablespace:
             tablespace_sql = self.connection.ops.tablespace_sql(model._meta.db_tablespace)
@@ -339,11 +340,15 @@ class BaseDatabaseSchemaEditor:
 
     def add_constraint(self, model, constraint):
         """Add a check constraint to a model."""
-        self.execute(constraint.create_sql(model, self))
+        sql = constraint.create_sql(model, self)
+        if sql:
+            self.execute(sql)
 
     def remove_constraint(self, model, constraint):
         """Remove a check constraint from a model."""
-        self.execute(constraint.remove_sql(model, self))
+        sql = constraint.remove_sql(model, self)
+        if sql:
+            self.execute(sql)
 
     def alter_unique_together(self, model, old_unique_together, new_unique_together):
         """
@@ -1008,7 +1013,14 @@ class BaseDatabaseSchemaEditor:
     def _delete_fk_sql(self, model, name):
         return self._delete_constraint_sql(self.sql_delete_fk, model, name)
 
-    def _unique_sql(self, fields, name):
+    def _unique_sql(self, model, fields, name, condition=None):
+        if condition:
+            # Databases support conditional unique constraints via a unique
+            # index.
+            sql = self._create_unique_sql(model, fields, name=name, condition=condition)
+            if sql:
+                self.deferred_sql.append(sql)
+            return None
         constraint = self.sql_unique_constraint % {
             'columns': ', '.join(map(self.quote_name, fields)),
         }
@@ -1017,7 +1029,7 @@ class BaseDatabaseSchemaEditor:
             'constraint': constraint,
         }
 
-    def _create_unique_sql(self, model, columns, name=None):
+    def _create_unique_sql(self, model, columns, name=None, condition=None):
         def create_unique_name(*args, **kwargs):
             return self.quote_name(self._create_index_name(*args, **kwargs))
 
@@ -1027,14 +1039,28 @@ class BaseDatabaseSchemaEditor:
         else:
             name = self.quote_name(name)
         columns = Columns(table, columns, self.quote_name)
-        return Statement(
-            self.sql_create_unique,
-            table=table,
-            name=name,
-            columns=columns,
-        )
+        if condition:
+            return Statement(
+                self.sql_create_unique_index,
+                table=table,
+                name=name,
+                columns=columns,
+                condition=' WHERE ' + condition,
+            ) if self.connection.features.supports_partial_indexes else None
+        else:
+            return Statement(
+                self.sql_create_unique,
+                table=table,
+                name=name,
+                columns=columns,
+            )
 
-    def _delete_unique_sql(self, model, name):
+    def _delete_unique_sql(self, model, name, condition=None):
+        if condition:
+            return (
+                self._delete_constraint_sql(self.sql_delete_index, model, name)
+                if self.connection.features.supports_partial_indexes else None
+            )
         return self._delete_constraint_sql(self.sql_delete_unique, model, name)
 
     def _check_sql(self, name, check):
diff --git a/django/db/backends/sqlite3/schema.py b/django/db/backends/sqlite3/schema.py
index e8e40bbe34..b1c91dba3c 100644
--- a/django/db/backends/sqlite3/schema.py
+++ b/django/db/backends/sqlite3/schema.py
@@ -4,6 +4,7 @@ from decimal import Decimal
 from django.apps.registry import Apps
 from django.db.backends.base.schema import BaseDatabaseSchemaEditor
 from django.db.backends.ddl_references import Statement
+from django.db.models import UniqueConstraint
 from django.db.transaction import atomic
 from django.db.utils import NotSupportedError
 
@@ -398,7 +399,13 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         self.delete_model(old_field.remote_field.through)
 
     def add_constraint(self, model, constraint):
-        self._remake_table(model)
+        if isinstance(constraint, UniqueConstraint) and constraint.condition:
+            super().add_constraint(model, constraint)
+        else:
+            self._remake_table(model)
 
     def remove_constraint(self, model, constraint):
-        self._remake_table(model)
+        if isinstance(constraint, UniqueConstraint) and constraint.condition:
+            super().remove_constraint(model, constraint)
+        else:
+            self._remake_table(model)
diff --git a/django/db/models/base.py b/django/db/models/base.py
index b55c5a32b1..faed79cc2f 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -1009,6 +1009,8 @@ class Model(metaclass=ModelBase):
         for model_class, model_constraints in constraints:
             for constraint in model_constraints:
                 if (isinstance(constraint, UniqueConstraint) and
+                        # Partial unique constraints can't be validated.
+                        constraint.condition is None and
                         not any(name in exclude for name in constraint.fields)):
                     unique_checks.append((model_class, constraint.fields))
 
diff --git a/django/db/models/constraints.py b/django/db/models/constraints.py
index 5b6f498d94..e7f81d3ee9 100644
--- a/django/db/models/constraints.py
+++ b/django/db/models/constraints.py
@@ -1,3 +1,4 @@
+from django.db.models.query_utils import Q
 from django.db.models.sql.query import Query
 
 __all__ = ['CheckConstraint', 'UniqueConstraint']
@@ -66,34 +67,55 @@ class CheckConstraint(BaseConstraint):
 
 
 class UniqueConstraint(BaseConstraint):
-    def __init__(self, *, fields, name):
+    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]
-        return schema_editor._unique_sql(fields, self.name)
+        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]
-        return schema_editor._create_unique_sql(model, fields, self.name)
+        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):
-        return schema_editor._delete_unique_sql(model, self.name)
+        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>' % (self.__class__.__name__, self.fields, self.name)
+        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
+            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