Features of a Custom List with External Data

Just like we looked at the limitations of a 100% External List, let’s look at the limitations of a custom list that uses as one of its columns an external data element.  Note that these observations apply no matter if you only have the one external data element or if you include additional fields from the external content type to include in your new list.  First, let’s get out of the way some things that are missing in a custom list when it includes external data.

1. Folders are still not allowed.  I’m sticking with the theory that when beginning with an external flat file (table) that there is no easy way to map the hierarchy concept of folders.
2. No version history.  Since SharePoint has no way to track changes to data in the backend database, version history does not make sense, no?
3. No workflows.  I suppose this limitation also relates to the inability to track changes to data in the external data source.
4. As with an External List, you cannot connect this custom list to Outlook.

Now let’s look at some of the things you can do with a custom list that you cannot do with the External list version of the same external content type.

1. You can open the custom list in Datasheet mode.  True, you cannot change any of the external data fields by just typing in new values, but you can update any of the custom fields created through SharePoint.
2. You can add more custom columns to the list and if you want, you can even edit the column that connects your custom list to the external content type to add or remove additional columns from the external data source.
3. You can export the custom list to Excel which will include not only the custom columns, but all of the selected columns from the External Content Type.  In fact, you could link your External Content Type to a minimal custom list with just the Title column to transfer all of the data to Excel.  You can also copy the custom list as a table in Access and it too will include the external data.  However, if you link the SharePoint list to the Access table you will not be able to edit the external data fields, but you can edit the other custom list field columns.
4. The custom list also lets you define an RSS feed for the list.  However the feed only contains the custom fields and the column used to link to the external content type.
5. You can attach a file to each custom list item.  This is true probably because the custom list has a physical presence in SharePoint while the External List which did not support attachments did not have a physical presence in which to store that link.
6. Item Permissions are allowed again due to a physical list presence from the custom columns of the custom list.
7. Finally, you can modify the list item edit form by using the InfoPath button on the List ribbon.  Interestingly, when InfoPath opens, it tells you that the connecting field from the external content type is not supported and will not be available.  If you click OK, the resulting form includes all of the custom fields you added to the list and the additional columns included from the external content type even though you will not be able to edit the external content type fields.  If you decide you no longer want to use the custom InfoPath form, you must go through List Settings à Form Settings (in the General Settings group) and select the radio button: Use the default SharePoint form.  If you do not click the Delete the InfoPath Form from the server, you will start from the existing form and will need to update all of the changed fields.