fix(populate): allow deselecting discriminator key when populating

Fix #3230
Re: #13760
Re: #13679

lib/helpers/projection/isExclusive.js

@@ -12,13 +12,12 @@ module.exports = function isExclusive(projection) {
   }
 
   const keys = Object.keys(projection);
-  let ki = keys.length;
   let exclude = null;
 
-  if (ki === 1 && keys[0] === '_id') {
+  if (keys.length === 1 && keys[0] === '_id') {
     exclude = !projection._id;
   } else {
-    while (ki--) {
+    for (let ki = 0; ki < keys.length; ++ki) {
       // Does this projection explicitly define inclusion/exclusion?
       // Explicitly avoid `$meta` and `$slice`
       const key = keys[ki];

lib/model.js

@@ -4408,7 +4408,7 @@ function populate(model, docs, options, callback) {
         select = select.replace(excludeIdRegGlobal, ' ');
       } else {
         // preserve original select conditions by copying
-        select = utils.object.shallowCopy(select);
+        select = { ...select };
         delete select._id;
       }
     }

lib/query.js

@@ -4932,16 +4932,14 @@ Query.prototype._castFields = function _castFields(fields) {
       elemMatchKeys,
       keys,
       key,
-      out,
-      i;
+      out;
 
   if (fields) {
     keys = Object.keys(fields);
     elemMatchKeys = [];
-    i = keys.length;
 
     // collect $elemMatch args
-    while (i--) {
+    for (let i = 0; i < keys.length; ++i) {
       key = keys[i];
       if (fields[key].$elemMatch) {
         selected || (selected = {});
@@ -4960,8 +4958,7 @@ Query.prototype._castFields = function _castFields(fields) {
     }
 
     // apply the casted field args
-    i = elemMatchKeys.length;
-    while (i--) {
+    for (let i = 0; i < elemMatchKeys.length; ++i) {
       key = elemMatchKeys[i];
       fields[key] = out[key];
     }

lib/queryhelpers.js

@@ -180,11 +180,12 @@ exports.applyPaths = function applyPaths(fields, schema) {
       if (!isDefiningProjection(field)) {
         continue;
       }
-      // `_id: 1, name: 0` is a mixed inclusive/exclusive projection in
-      // MongoDB 4.0 and earlier, but not in later versions.
       if (keys[keyIndex] === '_id' && keys.length > 1) {
         continue;
       }
+      if (keys[keyIndex] === schema.options.discriminatorKey && keys.length > 1 && field != null && !field) {
+        continue;
+      }
       exclude = !field;
       break;
     }

lib/utils.js

@@ -687,12 +687,6 @@ exports.object.vals = function vals(o) {
   return ret;
 };
 
-/**
- * @see exports.options
- */
-
-exports.object.shallowCopy = exports.options;
-
 const hop = Object.prototype.hasOwnProperty;
 
 /**

test/model.populate.test.js

@@ -10834,4 +10834,29 @@ describe('model: populate:', function() {
       [id1.toString(), id2.toString(), id3.toString(), id4.toString()]
     );
   });
+
+  it('allows deselecting discriminator key when populating (gh-3230) (gh-13760) (gh-13679)', async function() {
+    const Test = db.model(
+      'Test',
+      Schema({ name: String, arr: [{ testRef: { type: 'ObjectId', ref: 'Test2' } }] })
+    );
+
+    const schema = Schema({ name: String });
+    const Test2 = db.model('Test2', schema);
+    const D = Test2.discriminator('D', Schema({ prop: String }));
+
+
+    await Test.deleteMany({});
+    await Test2.deleteMany({});
+    const { _id } = await D.create({ name: 'foo', prop: 'bar' });
+    const test = await Test.create({ name: 'test', arr: [{ testRef: _id }] });
+
+    const doc = await Test
+      .findById(test._id)
+      .populate('arr.testRef', { name: 1, prop: 1, _id: 0, __t: 0 });
+    assert.deepStrictEqual(
+      doc.toObject().arr[0].testRef,
+      { name: 'foo', prop: 'bar' }
+    );
+  });
 });