add support for QuerySet.explain()

Author: timgraham

README.md

@@ -112,6 +112,16 @@ And whenever you run `python manage.py startapp`, you must remove the line:
 
 from the new application's `apps.py` file.
 
+## Notes on Django QuerySets
+
+* `QuerySet.explain()` supports the [`comment` and `verbosity` options](
+  https://www.mongodb.com/docs/manual/reference/command/explain/#command-fields).
+
+   Example: `QuerySet.explain(comment="...", verbosity="...")`
+
+   Valid values for `verbosity` are `"queryPlanner"` (default),
+   `"executionStats"`, and `"allPlansExecution"`.
+
 ## Known issues and limitations
 
 - The following `QuerySet` methods aren't supported:

django_mongodb/compiler.py

@@ -1,4 +1,5 @@
 import itertools
+import pprint
 from collections import defaultdict
 
 from bson import SON
@@ -491,6 +492,37 @@ def _get_ordering(self):
     def get_where(self):
         return self.where
 
+    def explain_query(self):
+        # Validate format (none supported) and options.
+        options = self.connection.ops.explain_query_prefix(
+            self.query.explain_info.format,
+            **self.query.explain_info.options,
+        )
+        # Build the query pipeline.
+        self.pre_sql_setup()
+        columns = self.get_columns()
+        query = self.build_query(
+            # Avoid $project (columns=None) if unneeded.
+            columns if self.query.annotations or not self.query.default_cols else None
+        )
+        pipeline = query.get_pipeline()
+        # Explain the pipeline.
+        kwargs = {}
+        for option in self.connection.ops.explain_options:
+            if value := options.get(option):
+                kwargs[option] = value
+        explain = self.connection.database.command(
+            "explain",
+            {"aggregate": self.collection_name, "pipeline": pipeline, "cursor": {}},
+            **kwargs,
+        )
+        # Generate the output: a list of lines that Django joins with newlines.
+        result = []
+        for key, value in explain.items():
+            formatted_value = pprint.pformat(value, indent=4)
+            result.append(f"{key}: {formatted_value}")
+        return result
+
 
 class SQLInsertCompiler(SQLCompiler):
     def execute_sql(self, returning_fields=None):

django_mongodb/features.py

@@ -8,6 +8,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     has_json_object_function = False
     has_native_json_field = True
     supports_date_lookup_using_string = False
+    supports_explaining_query_execution = True
     supports_foreign_keys = False
     supports_ignore_conflicts = False
     supports_json_field_contains = False
@@ -53,9 +54,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         "expressions.tests.IterableLookupInnerExpressionsTests.test_expressions_in_lookups_join_choice",
         # Unexpected alias_refcount in alias_map.
         "queries.tests.Queries1Tests.test_order_by_tables",
-        # QuerySet.explain() not implemented:
-        # https://github.com/mongodb-labs/django-mongodb/issues/28
-        "queries.test_explain.ExplainUnsupportedTests.test_message",
         # The $sum aggregation returns 0 instead of None for null.
         "aggregation.test_filter_argument.FilteredAggregateTests.test_plain_annotate",
         "aggregation.tests.AggregateTestCase.test_aggregation_default_passed_another_aggregate",

django_mongodb/operations.py

@@ -26,6 +26,7 @@ class DatabaseOperations(BaseDatabaseOperations):
         Combinable.BITOR: "bitOr",
         Combinable.BITXOR: "bitXor",
     }
+    explain_options = {"comment", "verbosity"}
 
     def adapt_datefield_value(self, value):
         """Store DateField as datetime."""
@@ -198,6 +199,19 @@ def _prep_lookup_value(self, value, field, field_kind, lookup):
             value = self.adapt_decimalfield_value(value, field.max_digits, field.decimal_places)
         return value
 
+    def explain_query_prefix(self, format=None, **options):
+        # Validate options.
+        validated_options = {}
+        if options:
+            for valid_option in self.explain_options:
+                value = options.pop(valid_option, None)
+                if value is not None:
+                    validated_options[valid_option] = value
+        # super() raises an error if any options are left after the valid ones
+        # are popped above.
+        super().explain_query_prefix(format, **options)
+        return validated_options
+
     """Django uses these methods to generate SQL queries before it generates MQL queries."""
 
     # EXTRACT format cannot be passed in parameters.