fix: support joining with children models

Author: Joel Mukuthu

lib/KnormRelations.js

@@ -97,7 +97,10 @@ class KnormRelations {
 
           for (const reference of asArray(references)) {
             // TODO: validate that reference is Field instance?
-            if (reference.model === Model) {
+            if (
+              reference.model === Model ||
+              Model.prototype instanceof reference.model
+            ) {
               resolvedReferences.push({
                 // TODO: Model.getField()
                 from: this._config._fields[fieldName],