Core Data: Bring back support for nested _fields values (#25083)

* Add a failing test demonstrating the issue

* Use lodash functions to check nested keys

* Fix undefined meta value

packages/core-data/src/queried-data/selectors.js

@@ -3,6 +3,7 @@
  */
 import createSelector from 'rememo';
 import EquivalentKeyMap from 'equivalent-key-map';
+import { get, has, set } from 'lodash';
 
 /**
  * Internal dependencies
@@ -76,12 +77,12 @@ function getQueriedItemsUncached( state, query ) {
 				// This accounts for the fact that queried items are stored by
 				// stable key without an associated fields query. Other requests
 				// may have included fewer fields properties.
-				const field = fields[ f ];
-				if ( ! item.hasOwnProperty( field ) ) {
+				const field = fields[ f ].split( '.' );
+				if ( ! has( item, field ) ) {
 					return null;
 				}
-
-				filteredItem[ field ] = item[ field ];
+				const value = get( item, field );
+				set( filteredItem, field, value );
 			}
 		} else {
 			// If expecting a complete item, validate that completeness, or

packages/core-data/src/queried-data/test/selectors.js

@@ -106,6 +106,47 @@ describe( 'getQueriedItems', () => {
 		] );
 	} );
 
+	it( 'should dynamically construct fields-filtered item from available data with nested fields', () => {
+		const state = {
+			items: {
+				1: {
+					id: 1,
+					content: 'chicken',
+					author: 'bob',
+					meta: {
+						template: 'single',
+						_private: 'unused',
+					},
+				},
+				2: {
+					id: 2,
+					content: 'ribs',
+					author: 'sally',
+					meta: {
+						template: 'single',
+						_private: 'unused',
+					},
+				},
+			},
+			itemIsComplete: {
+				1: true,
+				2: true,
+			},
+			queries: {
+				'': [ 1, 2 ],
+			},
+		};
+
+		const result = getQueriedItems( state, {
+			_fields: [ 'content', 'meta.template' ],
+		} );
+
+		expect( result ).toEqual( [
+			{ content: 'chicken', meta: { template: 'single' } },
+			{ content: 'ribs', meta: { template: 'single' } },
+		] );
+	} );
+
 	it( 'should return null if attempting to filter by yet-unknown fields', () => {
 		const state = {
 			items: {