diff --git a/apps/whale-api/src/module.database/database.spec/_model.ts b/apps/whale-api/src/module.database/database.spec/_model.ts
index b0e5a92fa8..219e1b7c0a 100644
--- a/apps/whale-api/src/module.database/database.spec/_model.ts
+++ b/apps/whale-api/src/module.database/database.spec/_model.ts
@@ -32,6 +32,7 @@ export const PartitionMapping: ModelMapping<Partition> = {
 }
 
 export interface Partition extends Model {
+  id: string
   a: string
   b: number
 
@@ -77,6 +78,7 @@ export const PartitionSortMapping: ModelMapping<PartitionSort> = {
 }
 
 export interface PartitionSort extends Model {
+  id: string
   a_partition: string
   a_sort: string
 
diff --git a/apps/whale-api/src/module.database/model.ts b/apps/whale-api/src/module.database/model.ts
index 0e5120931c..369c4fda3f 100644
--- a/apps/whale-api/src/module.database/model.ts
+++ b/apps/whale-api/src/module.database/model.ts
@@ -4,6 +4,16 @@
 export type ModelKey = string | number
 export type ModelKeyType = 'string' | 'number'
 
+interface ModelIndexString<M extends Model> {
+  type: 'string'
+  key: (model: M) => string
+}
+
+interface ModelIndexNumber<M extends Model> {
+  type: 'number'
+  key: (model: M) => number
+}
+
 /**
  * @see Model
  */
@@ -17,19 +27,14 @@ export interface ModelIndex<M extends Model> {
   /**
    * Partition key of the model index.
    */
-  partition: {
-    type: ModelKeyType
-    key: (model: M) => ModelKey
-  }
+  partition: ModelIndexString<M> | ModelIndexNumber<M>
+
   /**
    * Sort key of the model index, where present indicates composite key.
    * This attribute must be sorted in lexicographically order, which is a
    * typical implementation of most key-value store.
    */
-  sort?: {
-    type: ModelKeyType
-    key: (model: M) => ModelKey
-  }
+  sort?: ModelIndexString<M> | ModelIndexNumber<M>
 }
 
 export interface ModelMapping<M extends Model> {
diff --git a/apps/whale-api/src/module.database/provider.level/level.database.ts b/apps/whale-api/src/module.database/provider.level/level.database.ts
index 4dd9cfcd87..ac6bc9ecff 100644
--- a/apps/whale-api/src/module.database/provider.level/level.database.ts
+++ b/apps/whale-api/src/module.database/provider.level/level.database.ts
@@ -50,6 +50,9 @@ export abstract class LevelUpDatabase extends Database {
    * Sub root space for type.
    */
   protected subRoot<M extends Model> (mapping: ModelMapping<M>): LevelUp {
+    // TODO(fuxingloh): sub root indexing might dup too much indexed data as root indexes and sub indexes can be shared.
+    //  We could allow a sub index to act as root. Need to revisit this in the future.
+    //  Other providers like dynamodb where the indexes are manually setup it won't be such an issue.
     return sub(this.root, mapping.type, {
       valueEncoding: 'json',
       keyEncoding: 'binary'