Fix loopbackio#1795 - Count issue with related models using though model

lib/scope.js

@@ -517,11 +517,51 @@ function defineScope(cls, targetClass, name, params, methods, options) {
       options = {};
     }
     options = options || {};
-
+    // If there is a through model
+    // run another query to apply filter on relatedModel(targetModel)
+    // see github.com/strongloop/loopback-datasource-juggler/issues/1795
+    let scopeOnRelatedModel = false;
+    let queryRelated;
+    if (this._scope && this._scope.collect &&
+      where !== null && typeof where === 'object') {
+      queryRelated = {
+        relation: this._scope.collect,
+        scope: {
+          where: where,
+        },
+      };
+      where = {};
+      scopeOnRelatedModel = true;
+    }
     const targetModel = definition.targetModel(this._receiver);
     const scoped = (this._scope && this._scope.where) || {};
     const filter = mergeQuery({where: scoped}, {where: where || {}});
-    return targetModel.count(filter.where, options, cb);
+    if (!scopeOnRelatedModel) {
+      return targetModel.count(filter.where, options, cb);
+    }
+    return targetModel.find(filter, options, function(err, data) {
+      const relatedModel = targetModel.relations[queryRelated.relation].modelTo;
+      const keyFrom = targetModel.relations[queryRelated.relation].keyFrom;
+      const IdKey = idName(relatedModel);
+
+      // Merge queryRelated filter and targetId filter
+      const buildWhere = function() {
+        return {
+          and: [
+            {
+              [IdKey]: collectTargetIds(data, keyFrom || IdKey),
+            },
+            queryRelated.scope.where],
+        };
+      };
+      if (queryRelated.scope.where !== undefined) {
+        queryRelated.scope.where = buildWhere();
+      } else {
+        queryRelated.scope.where = {};
+        queryRelated.scope.where[IdKey] = collectTargetIds(data, keyFrom || IdKey);
+      }
+      return relatedModel.count(queryRelated.scope.where, options, cb);
+    });
   }
 
   return definition;