fix(query): handle deselecting _id when another field has schema-level select: false

lib/queryhelpers.js

@@ -163,6 +163,11 @@ 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;
+      }
       exclude = !field;
       break;
     }

test/model.field.selection.test.js

@@ -475,7 +475,6 @@ describe('model field selection', function() {
     db.deleteModel(/BlogPost/);
     const BlogPost = db.model('BlogPost', BlogPostSchema);
 
-
     await BlogPost.create({
       author: 'me',
       settings: {
@@ -497,8 +496,6 @@ describe('model field selection', function() {
   });
 
   it('when `select: true` in schema, works with $elemMatch in projection', async function() {
-
-
     const productSchema = new Schema({
       attributes: {
         select: true,
@@ -524,12 +521,10 @@ describe('model field selection', function() {
   });
 
   it('selection specified in query overwrites option in schema', async function() {
-
     const productSchema = new Schema({ name: { type: String, select: false } });
 
     const Product = db.model('Product', productSchema);
 
-
     await Product.create({ name: 'Computer' });
 
     const product = await Product.findOne().select('name');
@@ -538,7 +533,6 @@ describe('model field selection', function() {
   });
 
   it('selecting with `false` instead of `0` doesn\'t overwrite schema `select: false` (gh-8923)', async function() {
-
     const userSchema = new Schema({
       name: { type: String, select: false },
       age: { type: Number }
@@ -552,4 +546,21 @@ describe('model field selection', function() {
 
     assert.ok(!user.name);
   });
+
+  it('handles deselecting _id when other field has schema-level `select: false` (gh-12670)', async function() {
+    const schema = new mongoose.Schema({
+      field1: {
+        type: String,
+        select: false
+      },
+      field2: String
+    });
+    const User = db.model('User', schema);
+
+    await User.create({ field1: 'test1', field2: 'test2' });
+    const doc = await User.findOne().select('field2 -_id');
+    assert.ok(doc.field2);
+    assert.strictEqual(doc.field1, undefined);
+    assert.strictEqual(doc._id, undefined);
+  });
 });