diff --git a/TODO.md b/TODO.md
index 57dd05c1..f704c9a8 100644
--- a/TODO.md
+++ b/TODO.md
@@ -36,6 +36,7 @@
 - ~~Facet limit (hardcode to top 10)~~
 - ~~Deprecate old split function~~
 - ID should not have "/"
+- Use rocksdb batch put for atomic insertion
 - Test for sorted_array::indexOf when length is 0
 - Handle store-get() not finding a key
 - Fix API response codes
diff --git a/include/collection.h b/include/collection.h
index 4e95bb5f..8bac5741 100644
--- a/include/collection.h
+++ b/include/collection.h
@@ -62,7 +62,7 @@ private:
 
     std::string token_ordering_field;
 
-    std::string get_doc_id_key(std::string doc_id);
+    std::string get_doc_id_key(const std::string & doc_id);
 
     std::string get_seq_id_key(uint32_t seq_id);
 
@@ -114,9 +114,9 @@ public:
 
     ~Collection();
 
-    static std::string get_next_seq_id_key(std::string collection_name);
+    static std::string get_next_seq_id_key(const std::string & collection_name);
 
-    static std::string get_meta_key(std::string collection_name);
+    static std::string get_meta_key(const std::string & collection_name);
 
     std::string get_seq_id_collection_prefix();
 
@@ -134,20 +134,20 @@ public:
 
     std::string get_token_ordering_field();
 
-    Option<std::string> add(std::string json_str);
+    Option<std::string> add(const std::string & json_str);
 
     nlohmann::json search(std::string query, const std::vector<std::string> search_fields,
                           const std::string & simple_filter_query, const std::vector<std::string> & facet_fields,
                           const std::vector<sort_field> & sort_fields, const int num_typos,
                           const size_t num_results, const token_ordering token_order = FREQUENCY, const bool prefix = false);
 
-    Option<std::string> remove(std::string id);
+    Option<std::string> remove(const std::string & id);
 
     void score_results(const std::vector<sort_field> & sort_fields, const int & token_rank, Topster<100> &topster,
                        const std::vector<art_leaf *> & query_suggestion, const uint32_t *result_ids,
                        const size_t result_size) const;
 
-    Option<uint32_t> index_in_memory(const nlohmann::json &document, uint32_t seq_id);
+    Option<uint32_t> index_in_memory(const nlohmann::json & document, uint32_t seq_id);
 
     enum {MAX_SEARCH_TOKENS = 20};
     enum {MAX_RESULTS = 100};
diff --git a/src/collection.cpp b/src/collection.cpp
index 2055ef3a..ac220388 100644
--- a/src/collection.cpp
+++ b/src/collection.cpp
@@ -41,6 +41,7 @@ Collection::~Collection() {
 
     for(auto & name_map: sort_index) {
         delete name_map.second;
+        name_map.second = nullptr;
     }
 }
 
@@ -49,7 +50,7 @@ uint32_t Collection::get_next_seq_id() {
     return next_seq_id++;
 }
 
-Option<std::string> Collection::add(std::string json_str) {
+Option<std::string> Collection::add(const std::string & json_str) {
     nlohmann::json document = nlohmann::json::parse(json_str);
 
     uint32_t seq_id = get_next_seq_id();
@@ -294,7 +295,8 @@ void Collection::index_string_field(const std::string & text, const uint32_t sco
         }
 
         art_insert(t, key, key_len, &art_doc, num_hits);
-        delete art_doc.offsets;
+        delete [] art_doc.offsets;
+        art_doc.offsets = nullptr;
     }
 }
 
@@ -996,7 +998,7 @@ void Collection::remove_and_shift_offset_index(sorted_array &offset_index, const
     delete[] new_array;
 }
 
-Option<std::string> Collection::remove(std::string id) {
+Option<std::string> Collection::remove(const std::string & id) {
     std::string seq_id_str;
     StoreStatus status = store->get(get_doc_id_key(id), seq_id_str);
 
@@ -1112,7 +1114,7 @@ Option<std::string> Collection::remove(std::string id) {
     return Option<std::string>(id);
 }
 
-std::string Collection::get_next_seq_id_key(std::string collection_name) {
+std::string Collection::get_next_seq_id_key(const std::string & collection_name) {
     return std::string(COLLECTION_NEXT_SEQ_PREFIX) + "_" + collection_name;
 }
 
@@ -1127,7 +1129,7 @@ std::string Collection::get_seq_id_key(uint32_t seq_id) {
     return get_seq_id_collection_prefix() + "_" + std::string(bytes, bytes+4);
 }
 
-std::string Collection::get_doc_id_key(std::string doc_id) {
+std::string Collection::get_doc_id_key(const std::string & doc_id) {
     return std::to_string(collection_id) + "_" + DOC_ID_PREFIX + doc_id;
 }
 
@@ -1159,7 +1161,7 @@ spp::sparse_hash_map<std::string, field> Collection::get_schema() {
     return search_schema;
 };
 
-std::string Collection::get_meta_key(std::string collection_name) {
+std::string Collection::get_meta_key(const std::string & collection_name) {
     return COLLECTION_META_PREFIX + collection_name;
 }