[PoC] Push simple filter conditions into $lookup stage.

django_mongodb_backend/compiler.py

@@ -9,14 +9,16 @@
 from django.db.models.expressions import Case, Col, OrderBy, Ref, Value, When
 from django.db.models.functions.comparison import Coalesce
 from django.db.models.functions.math import Power
-from django.db.models.lookups import IsNull
+from django.db.models.lookups import IsNull, Lookup
 from django.db.models.sql import compiler
 from django.db.models.sql.constants import GET_ITERATOR_CHUNK_SIZE, MULTI, SINGLE
 from django.db.models.sql.datastructures import BaseTable
+from django.db.models.sql.where import AND
 from django.utils.functional import cached_property
 from pymongo import ASCENDING, DESCENDING
 
 from .query import MongoQuery, wrap_database_errors
+from .query_utils import is_direct_value
 
 
 class SQLCompiler(compiler.SQLCompiler):
@@ -549,10 +551,22 @@ def get_combinator_queries(self):
 
     def get_lookup_pipeline(self):
         result = []
+        where = self.get_where()
+        promote_filters = defaultdict(list)
+        for expr in where.children if where and where.connector == AND else ():
+            if (
+                isinstance(expr, Lookup)
+                and isinstance(expr.lhs, Col)
+                and (is_direct_value(expr.rhs) or isinstance(expr.rhs, Value))
+            ):
+                promote_filters[expr.lhs.alias].append(expr)
+
         for alias in tuple(self.query.alias_map):
             if not self.query.alias_refcount[alias] or self.collection_name == alias:
                 continue
-            result += self.query.alias_map[alias].as_mql(self, self.connection)
+            result += self.query.alias_map[alias].as_mql(
+                self, self.connection, promote_filters[alias]
+            )
         return result
 
     def _get_aggregate_expressions(self, expr):

django_mongodb_backend/query.py

@@ -129,25 +129,12 @@ def extra_where(self, compiler, connection):  # noqa: ARG001
     raise NotSupportedError("QuerySet.extra() is not supported on MongoDB.")
 
 
-def join(self, compiler, connection):
-    lookup_pipeline = []
-    lhs_fields = []
-    rhs_fields = []
-    # Add a join condition for each pair of joining fields.
-    parent_template = "parent__field__"
-    for lhs, rhs in self.join_fields:
-        lhs, rhs = connection.ops.prepare_join_on_clause(
-            self.parent_alias, lhs, compiler.collection_name, rhs
-        )
-        lhs_fields.append(lhs.as_mql(compiler, connection))
-        # In the lookup stage, the reference to this column doesn't include
-        # the collection name.
-        rhs_fields.append(rhs.as_mql(compiler, connection))
-    # Handle any join conditions besides matching field pairs.
-    extra = self.join_field.get_extra_restriction(self.table_alias, self.parent_alias)
-    if extra:
+def join(self, compiler, connection, pushed_expressions=None):
+    def _get_reroot_replacements(expressions):
+        if not expressions:
+            return None
         columns = []
-        for expr in extra.leaves():
+        for expr in expressions:
             # Determine whether the column needs to be transformed or rerouted
             # as part of the subquery.
             for hand_side in ["lhs", "rhs"]:
@@ -165,18 +152,47 @@ def join(self, compiler, connection):
         # based on their rerouted positions in the join pipeline.
         replacements = {}
         for col, parent_pos in columns:
-            column_target = Col(compiler.collection_name, expr.output_field.__class__())
+            target = col.target.clone()
+            target.remote_field = col.target.remote_field
+            column_target = Col(compiler.collection_name, target)
             if parent_pos is not None:
                 target_col = f"${parent_template}{parent_pos}"
                 column_target.target.db_column = target_col
                 column_target.target.set_attributes_from_name(target_col)
             else:
                 column_target.target = col.target
             replacements[col] = column_target
-        # Apply the transformed expressions in the extra condition.
+        return replacements
+
+    lookup_pipeline = []
+    lhs_fields = []
+    rhs_fields = []
+    # Add a join condition for each pair of joining fields.
+    parent_template = "parent__field__"
+    for lhs, rhs in self.join_fields:
+        lhs, rhs = connection.ops.prepare_join_on_clause(
+            self.parent_alias, lhs, compiler.collection_name, rhs
+        )
+        lhs_fields.append(lhs.as_mql(compiler, connection))
+        # In the lookup stage, the reference to this column doesn't include
+        # the collection name.
+        rhs_fields.append(rhs.as_mql(compiler, connection))
+    # Handle any join conditions besides matching field pairs.
+    extra = self.join_field.get_extra_restriction(self.table_alias, self.parent_alias)
+
+    if extra:
+        replacements = _get_reroot_replacements(extra.leaves())
         extra_condition = [extra.replace_expressions(replacements).as_mql(compiler, connection)]
     else:
         extra_condition = []
+    if self.join_type == INNER:
+        rerooted_replacement = _get_reroot_replacements(pushed_expressions)
+        resolved_pushed_expressions = [
+            expr.replace_expressions(rerooted_replacement).as_mql(compiler, connection)
+            for expr in pushed_expressions
+        ]
+    else:
+        resolved_pushed_expressions = []
 
     lookup_pipeline = [
         {
@@ -204,6 +220,7 @@ def join(self, compiler, connection):
                                     for i, field in enumerate(rhs_fields)
                                 ]
                                 + extra_condition
+                                + resolved_pushed_expressions
                             }
                         }
                     }

tests/queries_/test_mql.py

@@ -20,7 +20,8 @@ def test_join(self):
             "{'$lookup': {'from': 'queries__author', "
             "'let': {'parent__field__0': '$author_id'}, "
             "'pipeline': [{'$match': {'$expr': "
-            "{'$and': [{'$eq': ['$$parent__field__0', '$_id']}]}}}], 'as': 'queries__author'}}, "
+            "{'$and': [{'$eq': ['$$parent__field__0', '$_id']}, "
+            "{'$eq': ['$name', 'Bob']}]}}}], 'as': 'queries__author'}}, "
             "{'$unwind': '$queries__author'}, "
             "{'$match': {'$expr': {'$eq': ['$queries__author.name', 'Bob']}}}])",
         )