Add better error message re balderdashy/sails#4591

lib/waterline/methods/add-to-collection.js

@@ -373,6 +373,32 @@ module.exports = function addToCollection(/* targetRecordIds, collectionAttrName
             });
           }
 
+          // FUTURE: If anonymous junction model's primary key attribute is explicitly
+          // required, then this isn't going to work, because we're specifying
+          // a value for the primary key for the new junction records we're creating.
+          // We could, in waterline-schema (or possibly in sails-hook-orm or maybe
+          // even in Waterline core?), automatically un-require-ify the primary key
+          // attribute for anonymous junction models.
+          // > See https://github.com/balderdashy/sails/issues/4591 for background.
+          //
+          // But for now we just do this:
+          if (WLChild.junctionTable || WLChild.throughTable) {
+            if (WLChild.schema.id) {
+              if (WLChild.schema.id.required) {
+                throw new Error(
+                  'Cannot add to the collection for this many-to-many association because the anonymous '+
+                  'junction model\'s "id" (primary key) is required.  This might mean that the default id '+
+                  'in this app\'s `config/models.js` file makes all primary keys required.  For more info, '+
+                  'see https://github.com/balderdashy/sails/issues/4591.  If you are unsure, check out '+
+                  'https://sailsjs.com/support for help.'
+                );
+              }
+            } else {
+              // FUTURE: Maybe be smarter about this instead of just checking for `id`
+              // For now, we just ignore it and let the error happen.
+            }
+          }//fi
+
 
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