Allow specifying a specific constraint to use in ON CONFLICT

docs/source/conflict_handling.rst

@@ -87,6 +87,41 @@ Specifying multiple columns is necessary in case of a constraint that spans mult
    )
 
 
+Specific constraint
+*******************
+
+Alternatively, instead of specifying the columns the constraint you're targetting applies to, you can also specify the exact constraint to use:
+
+.. code-block:: python
+
+   from django.db import models
+   from psqlextra.models import PostgresModel
+
+   class MyModel(PostgresModel)
+       class Meta:
+           constraints = [
+               models.UniqueConstraint(
+                   name="myconstraint",
+                   fields=["first_name", "last_name"]
+               ),
+           ]
+
+       first_name = models.CharField(max_length=255)
+       last_name = models.CharField(max_length=255)
+
+   constraint = next(
+       constraint
+       for constraint in MyModel._meta.constraints
+       if constraint.name == "myconstraint"
+    ), None)
+
+   obj = (
+       MyModel.objects
+       .on_conflict(constraint, ConflictAction.UPDATE)
+       .insert_and_get(first_name='Henk', last_name='Jansen')
+   )
+
+
 HStore keys
 ***********
 Catching conflicts in columns with a ``UNIQUE`` constraint on a :class:`~psqlextra.fields.HStoreField` key is also supported:

psqlextra/compiler.py

@@ -243,11 +243,11 @@ def _rewrite_insert_on_conflict(
 
         # build the conflict target, the columns to watch
         # for conflicts
-        conflict_target = self._build_conflict_target()
+        on_conflict_clause = self._build_on_conflict_clause()
         index_predicate = self.query.index_predicate  # type: ignore[attr-defined]
         update_condition = self.query.conflict_update_condition  # type: ignore[attr-defined]
 
-        rewritten_sql = f"{sql} ON CONFLICT {conflict_target}"
+        rewritten_sql = f"{sql} {on_conflict_clause}"
 
         if index_predicate:
             expr_sql, expr_params = self._compile_expression(index_predicate)
@@ -270,6 +270,21 @@ def _rewrite_insert_on_conflict(
 
         return (rewritten_sql, params)
 
+    def _build_on_conflict_clause(self):
+        if django.VERSION >= (2, 2):
+            from django.db.models.constraints import BaseConstraint
+            from django.db.models.indexes import Index
+
+            if isinstance(
+                self.query.conflict_target, BaseConstraint
+            ) or isinstance(self.query.conflict_target, Index):
+                return "ON CONFLICT ON CONSTRAINT %s" % self.qn(
+                    self.query.conflict_target.name
+                )
+
+        conflict_target = self._build_conflict_target()
+        return f"ON CONFLICT {conflict_target}"
+
     def _build_conflict_target(self):
         """Builds the `conflict_target` for the ON CONFLICT clause."""
 

psqlextra/query.py

@@ -20,7 +20,11 @@
 from .sql import PostgresInsertQuery, PostgresQuery
 from .types import ConflictAction
 
-ConflictTarget = List[Union[str, Tuple[str]]]
+if TYPE_CHECKING:
+    from django.db.models.constraints import BaseConstraint
+    from django.db.models.indexes import Index
+
+ConflictTarget = Union[List[Union[str, Tuple[str]]], "BaseConstraint", "Index"]
 
 
 TModel = TypeVar("TModel", bound=models.Model, covariant=True)

tests/test_on_conflict_update.py

@@ -1,3 +1,4 @@
+import django
 import pytest
 
 from django.db import models
@@ -41,6 +42,35 @@ def test_on_conflict_update():
     assert obj2.cookies == "choco"
 
 
+@pytest.mark.skipif(
+    django.VERSION < (2, 2),
+    reason="Django < 2.2 doesn't implement constraints",
+)
+def test_on_conflict_update_by_unique_constraint():
+    model = get_fake_model(
+        {
+            "title": models.CharField(max_length=255, null=True),
+        },
+        meta_options={
+            "constraints": [
+                models.UniqueConstraint(name="test_uniq", fields=["title"]),
+            ],
+        },
+    )
+
+    constraint = next(
+        (
+            constraint
+            for constraint in model._meta.constraints
+            if constraint.name == "test_uniq"
+        )
+    )
+
+    model.objects.on_conflict(constraint, ConflictAction.UPDATE).insert_and_get(
+        title="title"
+    )
+
+
 def test_on_conflict_update_foreign_key_by_object():
     """Tests whether simple upsert works correctly when the conflicting field
     is a foreign key specified as an object."""