Merge pull request #15720 from Automattic/vkarpov15/gh-15718

fix(schema): avoid throwing error on array of unions

Author: vkarpov15

lib/schema.js

@@ -1694,6 +1694,10 @@ Schema.prototype.interpretAsType = function(path, obj, options) {
           `\`${name}\` is not a valid type within the array \`${path}\`.` +
           'See https://bit.ly/mongoose-schematypes for a list of valid schema types.');
       }
+
+      if (name === 'Union' && typeof cast === 'object') {
+        cast.parentSchema = this;
+      }
     }
 
     return new MongooseTypes.Array(path, cast || MongooseTypes.Mixed, obj, options);

test/schema.union.test.js

@@ -134,4 +134,28 @@ describe('Union', function() {
     const doc2FromDb = await TestModel.collection.findOne({ _id: doc2._id });
     assert.strictEqual(doc2FromDb.test, 'bbb');
   });
+
+  it('handles arrays of unions (gh-15718)', async function() {
+    const schema = new Schema({
+      arr: [{
+        type: 'Union',
+        of: [Number, Date]
+      }]
+    });
+    const TestModel = db.model('Test', schema);
+
+    const numValue = 42;
+    const dateValue = new Date('2025-06-01');
+
+    const doc = new TestModel({
+      arr: [numValue, dateValue]
+    });
+
+    await doc.save();
+
+    const found = await TestModel.collection.findOne({ _id: doc._id });
+    assert.strictEqual(found.arr.length, 2);
+    assert.strictEqual(found.arr[0], numValue);
+    assert.strictEqual(new Date(found.arr[1]).valueOf(), dateValue.valueOf());
+  });
 });