fix(resolver): it now yields the correct dataset for last+before args

Target the issue #7
In backward situations with cursors, the resolver uses count to compute
the "skip" value to forward to the findMany resolver. If the count value
is the global one, and not the one with the rawQuery with the '$lt'
statement, the value is incorrect. Hence the invalid dataset.

Author: Fanaen

src/__tests__/connectionResolver-test.js

@@ -598,6 +598,19 @@ describe('connectionResolver', () => {
         expect(conn.edges[0].node.id).toBe(result.edges[2].node.id);
       });
     });
+
+    it('should return previous edges when querying with last and before', async () => {
+      const result = await connectionResolver.resolve({
+        args: {
+          sort: sortOptions.ID_ASC.value,
+          before: dataToCursor({ id: 4 }),
+          last: 2,
+        },
+      });
+      expect(result.edges).toHaveLength(2);
+      expect(result.edges[0].node.id).toBe(2);
+      expect(result.edges[1].node.id).toBe(3);
+    });
   });
 
   describe('how works filter argument with resolve', () => {

src/connectionResolver.js

@@ -187,18 +187,30 @@ export function prepareConnectionResolver<TSource, TContext>(
         findManyParams.projection = { ...projection };
       }
 
+      // Apply the rawQuery to the count to get accurate results with last and before
+      const sortConfig: ?ConnectionSortOpts = findSortConfig(opts.sort, args.sort);
+      if (sortConfig) {
+        prepareRawQuery(resolveParams, sortConfig);
+      }
+
       if (!first && last) {
-        first = await countPromise;
+        // Get the number of edges targeted by the findMany resolver (not the whole count)
+        const filteredCountParams: $Shape<ResolveParams<TSource, TContext>> = {
+          ...resolveParams,
+          args: {
+            filter: { ...resolveParams.args.filter },
+          },
+        };
+
+        first = await countResolve(filteredCountParams);
         first = parseInt(first, 10) || 0;
       }
 
       let limit = last || first || 20;
       let skip = last > 0 ? first - last : 0;
 
       let prepareCursorData;
-      const sortConfig: ?ConnectionSortOpts = findSortConfig(opts.sort, args.sort);
       if (sortConfig) {
-        prepareRawQuery(resolveParams, sortConfig);
         findManyParams.rawQuery = resolveParams.rawQuery;
         sortConfig.cursorFields.forEach(fieldName => {
           findManyParams.projection[fieldName] = true;