database – MongoDb vs ElasticSearch for read operations?

We are contemplating using ElasticSearch for ALL read operations. And mongoDb as a database for write operations. What are your views on it? We do not have a requirement of full text search as such. But what we do have is complicated queries that could involve multiple collections and various operations such as lookup(join), group by, filter criteria etc.

How do Elasticsearch query language/capabilities compare against MongoDb?