If you are creating an App for SharePoint 2013 and plan to access BCS external data by using the REST API to query an external list, keep in mind that you must package your External Content Type in the SPAPP as well. If you do not, you will see this error returned by your request to the REST service:
Access denied. You do not have permission to perform this action or access this resource.
And you will see this error in the ULS logs when the query attempts to execute.
System.UnauthorizedAccessException: Apps cannot use External Content Types defined in the Business Data Connectivity Shared Service Application metadata catalog. Apps should package any External Content Type definitions in a BDC model file which is packaged and deployed as part of the SharePoint App.
The metadata catalog file BusinessDataMetadataCatalog/BDCMetadata.bdcm could not be found on the site http://app-cbb5c08e54bb2a.apps.powder.ski/BCSExternalDataSPApp.
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