fix: remove discriminator schema _id before merging if base schema has custom _id without calling merge()

Fix #8546
Re: #8543

Author: vkarpov15

lib/helpers/model/discriminator.js

@@ -72,8 +72,7 @@ module.exports = function discriminator(model, name, schema, tiedValue, applyPlu
     if (baseSchema.paths._id &&
         baseSchema.paths._id.options &&
         !baseSchema.paths._id.options.auto) {
-      delete schema.paths._id;
-      delete schema.tree._id;
+      schema.remove('_id');
     }
 
     // Find conflicting paths: if something is a path in the base schema
@@ -88,7 +87,7 @@ module.exports = function discriminator(model, name, schema, tiedValue, applyPlu
     }
 
     utils.merge(schema, baseSchema, {
-      omit: { discriminators: true },
+      omit: { discriminators: true, base: true },
       omitNested: conflictingPaths.reduce((cur, path) => {
         cur['tree.' + path] = true;
         return cur;

test/model.discriminator.test.js

@@ -1333,7 +1333,7 @@ describe('model', function() {
         const model = mongoose.model('Model', new mongoose.Schema({ _id: Number }));
         const passedInSchema = new mongoose.Schema({});
         model.discriminator('Discrimintaor', passedInSchema);
-        assert.equal(passedInSchema.path('_id').instance, 'ObjectID');
+        assert.equal(passedInSchema.path('_id').instance, 'Number');
       });
 
       function throwErrorOnClone() { throw new Error('clone() was called on the unrelated schema'); };