diff --git a/lib/model.js b/lib/model.js
index a9a0784c50e..a1a96ebf6b8 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -1337,23 +1337,8 @@ Model.syncIndexes = function syncIndexes(options, callback) {
         }
 
         for (const schemaIndex of schemaIndexes) {
-          const key = schemaIndex[0];
-          const options = _decorateDiscriminatorIndexOptions(this,
-            utils.clone(schemaIndex[1]));
-
-          // If these options are different, need to rebuild the index
-          const optionKeys = ['unique', 'partialFilterExpression', 'sparse', 'expireAfterSeconds'];
-          const indexCopy = Object.assign({}, index);
-          for (const key of optionKeys) {
-            if (!(key in options) && !(key in indexCopy)) {
-              continue;
-            }
-            indexCopy[key] = options[key];
-          }
-          if (utils.deepEqual(key, index.key) &&
-              utils.deepEqual(index, indexCopy)) {
+          if (isIndexEqual(this, schemaIndex, index)) {
             found = true;
-            break;
           }
         }
 
@@ -1406,6 +1391,43 @@ Model.syncIndexes = function syncIndexes(options, callback) {
   }, this.events);
 };
 
+/*!
+ * ignore
+ */
+
+function isIndexEqual(model, schemaIndex, dbIndex) {
+  const key = schemaIndex[0];
+  const options = _decorateDiscriminatorIndexOptions(model,
+    utils.clone(schemaIndex[1]));
+
+  // If these options are different, need to rebuild the index
+  const optionKeys = ['unique', 'partialFilterExpression', 'sparse', 'expireAfterSeconds'];
+  for (const key of optionKeys) {
+    if (!(key in options) && !(key in dbIndex)) {
+      continue;
+    }
+    if (!utils.deepEqual(options[key], dbIndex[key])) {
+      return false;
+    }
+  }
+
+  const schemaIndexKeys = Object.keys(key);
+  const dbIndexKeys = Object.keys(dbIndex.key);
+  if (schemaIndexKeys.length !== dbIndexKeys.length) {
+    return false;
+  }
+  for (let i = 0; i < schemaIndexKeys.length; ++i) {
+    if (schemaIndexKeys[i] !== dbIndexKeys[i]) {
+      return false;
+    }
+    if (!utils.deepEqual(key[schemaIndexKeys[i]], dbIndex.key[dbIndexKeys[i]])) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
 /**
  * Lists the indexes currently defined in MongoDB. This may or may not be
  * the same as the indexes defined in your schema depending on whether you