IdentityMap
This section is relevant only if you are using Mongoid 3. IdentityMap has been removed in Mongoid 4 because it now includes Mongoid::Relations::Eager.
An IdentityMap pattern is a design pattern where the object gets loaded only once from the database, and is then stored in the map. So, the next lookup is directly from the map and hence faster. The IdentityMap pattern has been implemented in Mongoid (and removed in the latest version).
Note
Ignore the rest of this section if you use Mongoid4. Only if you are using Mongoid 3, is the remaining section relevant.
If enabled properly, IdentityMap can greatly improve the performance of queries. Whenever a document is loaded from the database, it is classified by class and ID kept in the IdentityMap pattern. Then, when that document is looked up the next time, Mongoid looks into the IdentityMap pattern before looking into the database.
So, how do we know that the IdentityMap pattern is there? First, enable it.
# config/mongoid.yml development...
