We need to be able to mashup data online with enterprise data in a SSAS cube. To do this, we'll need the data modelling feature accessible when connecting to SSAS Tabular.
This is indeed a key missing feature and hard to understand why Excel can connect to multiple sources likes SSAS and other, but PowerBI cannot. It might become a kind of complex problem under the hood, but in my case it would be sufficient to render the different data sources on different Report pages. May be this is easier to accomplish?
