documented the fact that prefetch is not allowed on mixins

README.md

@@ -245,6 +245,9 @@ And you can also traverse the reverse relationship from a `Note` to its `annotat
 
 Note that `annotated` is a polymorphic relationship as it may yield either a `Project` 
 or a `Task` (or any other entity which is `Annotatable').
+
+Note: Prefetch is not allowed on a relationship that targets a mixin. In the example above
+you cannot prefetch the `annotated` relationship when querying the `Note` entity.
 
 Notes: this feature is very experimental at this stage. It needs more testing.
   Support for "is a" relationships (classical inheritence) is also in the works.

lib/persistence.js

@@ -1531,6 +1531,7 @@ persistence.get = function(arg1, arg2) {
     /**
      * Returns a new query collection which will prefetch a certain object relationship.
      * Only works with 1:1 and N:1 relations.
+     * Relation must target an entity, not a mix-in.
      * @param rel the relation name of the relation to prefetch
      * @return the query collection prefetching `rel`
      */

lib/persistence.store.sql.js

@@ -615,6 +615,8 @@ persistence.store.sql.config = function(persistence, dialect) {
     for ( var i = 0; i < this._prefetchFields.length; i++) {
       var prefetchField = this._prefetchFields[i];
       var thisMeta = meta.hasOne[prefetchField].type.meta;
+      if (thisMeta.isMixin)
+        throw new Error("cannot prefetch a mixin");
       var tableAlias = thisMeta.name + '_' + prefetchField + "_tbl";
       selectFields = selectFields.concat(selectAll(thisMeta, tableAlias,
           prefetchField + "_"));