feat(entity): add selector to select entity by id

* add a new selector 'selectById' to EntitySelector (model)
* implement selector, function to take id string || number
* add tests

See: ngrx#1156

modules/entity/spec/state_selectors.spec.ts

@@ -55,6 +55,12 @@ describe('Entity State Selectors', () => {
 
       expect(total).toEqual(3);
     });
+
+    it('should create a selector for selecting an entity by id', () => {
+      const entity = selectors.selectById('tgg')(state);
+
+      expect(entity).toEqual(TheGreatGatsby);
+    });
   });
 
   describe('Uncomposed Selectors', () => {
@@ -100,5 +106,11 @@ describe('Entity State Selectors', () => {
 
       expect(total).toEqual(3);
     });
+
+    it('should create a selector for selecting an entity by id', () => {
+      const entity = selectors.selectById('tgg')(state);
+
+      expect(entity).toEqual(TheGreatGatsby);
+    });
   });
 });

modules/entity/src/models.ts

@@ -73,6 +73,7 @@ export type EntitySelectors<T, V> = {
   selectEntities: (state: V) => Dictionary<T>;
   selectAll: (state: V) => T[];
   selectTotal: (state: V) => number;
+  selectById: (state: V) => T;
 };
 
 export interface EntityAdapter<T> extends EntityStateAdapter<T> {

modules/entity/src/state_selectors.ts

@@ -17,6 +17,8 @@ export function createSelectorsFactory<T>() {
       (ids: T[], entities: Dictionary<T>): any =>
         ids.map((id: any) => (entities as any)[id])
     );
+    const selectById = (id: string | number) =>
+      createSelector(selectEntities, (entities: Dictionary<T>) => entities[id]);
 
     const selectTotal = createSelector(selectIds, ids => ids.length);
 
@@ -26,6 +28,7 @@ export function createSelectorsFactory<T>() {
         selectEntities,
         selectAll,
         selectTotal,
+        selectById,
       };
     }
 
@@ -34,6 +37,8 @@ export function createSelectorsFactory<T>() {
       selectEntities: createSelector(selectState, selectEntities),
       selectAll: createSelector(selectState, selectAll),
       selectTotal: createSelector(selectState, selectTotal),
+      selectById: (id: string | number) =>
+        createSelector(selectState, selectById(id)),
     };
   }