fix(query): propagate read concern to populate if readConcern() called after populate()

Re: #15553

Author: vkarpov15

lib/query.js

@@ -2133,13 +2133,25 @@ Query.prototype._optionsForExec = function(model) {
     }
   }
 
-  if (this._mongooseOptions.populate && options.readPreference) {
-    for (const pop of Object.values(this._mongooseOptions.populate)) {
-      if (pop.options?.readPreference === undefined) {
-        if (!pop.options) {
-          pop.options = {};
+  if (this._mongooseOptions.populate) {
+    if (options.readPreference) {
+      for (const pop of Object.values(this._mongooseOptions.populate)) {
+        if (pop.options?.readPreference === undefined) {
+          if (!pop.options) {
+            pop.options = {};
+          }
+          pop.options.readPreference = options.readPreference;
+        }
+      }
+    }
+    if (options.readConcern) {
+      for (const pop of Object.values(this._mongooseOptions.populate)) {
+        if (pop.options?.readConcern === undefined) {
+          if (!pop.options) {
+            pop.options = {};
+          }
+          pop.options.readConcern = options.readConcern;
         }
-        pop.options.readPreference = options.readPreference;
       }
     }
   }
@@ -4929,18 +4941,6 @@ Query.prototype.populate = function() {
 
   const res = utils.populate.apply(null, args);
 
-  // Propagate readConcern from parent query, unless one already specified. Read pref propagated separately
-  if (this.options != null) {
-    const readConcern = this.options.readConcern;
-
-    for (const populateOptions of res) {
-      if (readConcern != null && (populateOptions && populateOptions.options && populateOptions.options.readConcern) == null) {
-        populateOptions.options = populateOptions.options || {};
-        populateOptions.options.readConcern = readConcern;
-      }
-    }
-  }
-
   const opts = this._mongooseOptions;
 
   if (opts.lean != null) {

test/query.test.js

@@ -4492,6 +4492,29 @@ describe('Query', function() {
     assert.strictEqual(query._mongooseOptions.populate.friends.options.readPreference, 'secondaryPreferred');
   });
 
+  it('propagates readConcern to populate options if readConcern() is called after populate() (gh-15553)', async function() {
+    const schema = new Schema({ name: String, age: Number, friends: [{ type: 'ObjectId', ref: 'Person' }] });
+    const Person = db.model('Person', schema);
+
+    let query = Person.find({}).populate('friends');
+    query.readConcern('majority');
+    await query.exec();
+    assert.strictEqual(query._mongooseOptions.populate.friends.options.readConcern.level, 'majority');
+
+    query = Person.find({}).readConcern('local').populate('friends');
+    query.readConcern('majority');
+    await query.exec();
+    assert.strictEqual(query._mongooseOptions.populate.friends.options.readConcern.level, 'majority');
+
+    query = Person.find({}).readConcern('majority').populate('friends');
+    await query.exec();
+    assert.strictEqual(query._mongooseOptions.populate.friends.options.readConcern.level, 'majority');
+
+    query = Person.find({}).readConcern('majority').populate({ path: 'friends', options: { readConcern: 'local' } });
+    await query.exec();
+    assert.strictEqual(query._mongooseOptions.populate.friends.options.readConcern, 'local');
+  });
+
   describe('Query with requireFilter', function() {
     let Person;
     let _id;