Add Index.get_pymongo_index_model() hook

django_mongodb_backend/indexes.py

@@ -1,8 +1,12 @@
+from collections import defaultdict
+
 from django.db import NotSupportedError
 from django.db.models import Index
 from django.db.models.lookups import BuiltinLookup
 from django.db.models.sql.query import Query
 from django.db.models.sql.where import AND, XOR, WhereNode
+from pymongo import ASCENDING, DESCENDING
+from pymongo.operations import IndexModel
 
 from .query_utils import process_rhs
 
@@ -36,6 +40,45 @@ def builtin_lookup_idx(self, compiler, connection):
     return {lhs_mql: {operator: value}}
 
 
+def get_pymongo_index_model(self, model, schema_editor, field=None, unique=False, column_prefix=""):
+    """Return a pymongo IndexModel for this Django Index."""
+    if self.contains_expressions:
+        return None
+    kwargs = {}
+    filter_expression = defaultdict(dict)
+    if self.condition:
+        filter_expression.update(self._get_condition_mql(model, schema_editor))
+    if unique:
+        kwargs["unique"] = True
+        # Indexing on $type matches the value of most SQL databases by
+        # allowing multiple null values for the unique constraint.
+        if field:
+            column = column_prefix + field.column
+            filter_expression[column].update({"$type": field.db_type(schema_editor.connection)})
+        else:
+            for field_name, _ in self.fields_orders:
+                field_ = model._meta.get_field(field_name)
+                filter_expression[field_.column].update(
+                    {"$type": field_.db_type(schema_editor.connection)}
+                )
+    if filter_expression:
+        kwargs["partialFilterExpression"] = filter_expression
+    index_orders = (
+        [(column_prefix + field.column, ASCENDING)]
+        if field
+        else [
+            # order is "" if ASCENDING or "DESC" if DESCENDING (see
+            # django.db.models.indexes.Index.fields_orders).
+            (
+                column_prefix + model._meta.get_field(field_name).column,
+                ASCENDING if order == "" else DESCENDING,
+            )
+            for field_name, order in self.fields_orders
+        ]
+    )
+    return IndexModel(index_orders, name=self.name, **kwargs)
+
+
 def where_node_idx(self, compiler, connection):
     if self.connector == AND:
         operator = "$and"
@@ -61,4 +104,5 @@ def where_node_idx(self, compiler, connection):
 def register_indexes():
     BuiltinLookup.as_mql_idx = builtin_lookup_idx
     Index._get_condition_mql = _get_condition_mql
+    Index.get_pymongo_index_model = get_pymongo_index_model
     WhereNode.as_mql_idx = where_node_idx

django_mongodb_backend/schema.py

@@ -1,9 +1,5 @@
-from collections import defaultdict
-
 from django.db.backends.base.schema import BaseDatabaseSchemaEditor
 from django.db.models import Index, UniqueConstraint
-from pymongo import ASCENDING, DESCENDING
-from pymongo.operations import IndexModel
 
 from .fields import EmbeddedModelField
 from .query import wrap_database_errors
@@ -264,43 +260,12 @@ def alter_unique_together(
     def add_index(
         self, model, index, *, field=None, unique=False, column_prefix="", parent_model=None
     ):
-        if index.contains_expressions:
-            return
-        kwargs = {}
-        filter_expression = defaultdict(dict)
-        if index.condition:
-            filter_expression.update(index._get_condition_mql(model, self))
-        if unique:
-            kwargs["unique"] = True
-            # Indexing on $type matches the value of most SQL databases by
-            # allowing multiple null values for the unique constraint.
-            if field:
-                column = column_prefix + field.column
-                filter_expression[column].update({"$type": field.db_type(self.connection)})
-            else:
-                for field_name, _ in index.fields_orders:
-                    field_ = model._meta.get_field(field_name)
-                    filter_expression[field_.column].update(
-                        {"$type": field_.db_type(self.connection)}
-                    )
-        if filter_expression:
-            kwargs["partialFilterExpression"] = filter_expression
-        index_orders = (
-            [(column_prefix + field.column, ASCENDING)]
-            if field
-            else [
-                # order is "" if ASCENDING or "DESC" if DESCENDING (see
-                # django.db.models.indexes.Index.fields_orders).
-                (
-                    column_prefix + model._meta.get_field(field_name).column,
-                    ASCENDING if order == "" else DESCENDING,
-                )
-                for field_name, order in index.fields_orders
-            ]
+        idx = index.get_pymongo_index_model(
+            model, schema_editor=self, field=field, unique=unique, column_prefix=column_prefix
         )
-        idx = IndexModel(index_orders, name=index.name, **kwargs)
-        model = parent_model or model
-        self.get_collection(model._meta.db_table).create_indexes([idx])
+        if idx:
+            model = parent_model or model
+            self.get_collection(model._meta.db_table).create_indexes([idx])
 
     def _add_composed_index(self, model, field_names, column_prefix="", parent_model=None):
         """Add an index on the given list of field_names."""