An association in the OGM sense is stored in a table (or rather is the query result of what would be retrieved from a table). So I don't think the notion of A vs E:TableName makes much sense. Ora rather I don't like this flat collapsing. But on the other hand I feel like a category / taxonomy additional field would be too much.

Really the more natural mapping is to have the association info in the entity document. And in that situation everything is clear.