PYTHON-1853 Empty projections should return the entire document not just the _id (#738)

doc/changelog.rst

@@ -143,6 +143,13 @@ Breaking Changes in 4.0
   opposed to
   the previous syntax which was simply ``if collection:`` or ``if database:``.
   You must now explicitly compare with None.
+- Empty projections (eg {} or []) for
+  :meth:`~pymongo.collection.Collection.find`, and
+  :meth:`~pymongo.collection.Collection.find_one`
+  are passed to the server as-is rather than the previous behavior which
+  substituted in a projection of ``{"_id": 1}``. This means that an empty
+  projection will now return the entire document, not just the ``"_id"`` field.
+
 
 Notable improvements
 ....................

doc/migrate-to-pymongo4.rst

@@ -637,6 +637,22 @@ Can be changed to this::
 
 You must now explicitly compare with None.
 
+Collection.find returns entire document with empty projection
+.............................................................
+Empty projections (eg {} or []) for
+:meth:`~pymongo.collection.Collection.find`, and
+:meth:`~pymongo.collection.Collection.find_one`
+are passed to the server as-is rather than the previous behavior which
+substituted in a projection of ``{"_id": 1}``. This means that an empty
+projection will now return the entire document, not just the ``"_id"`` field.
+To ensure that behavior remains consistent, code like this::
+
+  coll.find({}, projection={})
+
+Can be changed to this::
+
+  coll.find({}, projection={"_id":1})
+
 SONManipulator is removed
 -------------------------
 

pymongo/collection.py

@@ -1246,6 +1246,10 @@ class Collection(common.BaseObject):
 
         .. versionchanged:: 4.0
            Removed the ``modifiers`` option.
+           Empty projections (eg {} or []) are passed to the server as-is,
+           rather than the previous behavior which substituted in a
+           projection of ``{"_id": 1}``. This means that an empty projection
+           will now return the entire document, not just the ``"_id"`` field.
 
         .. versionchanged:: 3.11
            Added the ``allow_disk_use`` option.

pymongo/cursor.py

@@ -195,8 +195,6 @@ class Cursor(object):
             allow_disk_use = validate_boolean("allow_disk_use", allow_disk_use)
 
         if projection is not None:
-            if not projection:
-                projection = {"_id": 1}
             projection = helpers._fields_list_to_dict(projection, "projection")
 
         self.__spec = spec

test/test_collection.py

@@ -1707,7 +1707,10 @@ class TestCollection(IntegrationTest):
         self.assertTrue("hello" in db.test.find_one(projection=frozenset(["hello"])))
         self.assertTrue("hello" not in db.test.find_one(projection=frozenset(["foo"])))
 
-        self.assertEqual(["_id"], list(db.test.find_one(projection=[])))
+        self.assertEqual(["_id"], list(db.test.find_one(projection={'_id':
+            True})))
+        self.assertTrue("hello" in list(db.test.find_one(projection={})))
+        self.assertTrue("hello" in list(db.test.find_one(projection=[])))
 
         self.assertEqual(None, db.test.find_one({"hello": "foo"}))
         self.assertEqual(None, db.test.find_one(ObjectId()))