Combining data sources from NoSQL and SQL systems leads to data distribution and complexifies user queries: data is distributed among different stores having different data models. This data implementation complexifies the writing of user queries. This work proposes a querying framework of a polystore with the use of unified models as a user vision of the polystore. Unified models hides the variety of data models and data distribution to the user. Our solution uses the Entity-Relationship model of the polystore to infer unified models and to identify intermediate required operations to fulfill querying on real polystore. Using these required transformations, a rewriting framework allows to automatically rewrite the user query (against the unified model) with respect to the real data distribution over the polystore. We apply this framework with one dataset (UniBench benchmark) between a relational, a document-oriented and a graph-oriented databases. We illustrate in this work performance and the low impact of our query rewriting solution when compared to query execution time.