Create an IdentityMap for each entity

[vartanbeno]

The identity map design pattern is a means of improving performance by using an in-memory cache to prevent multiple retrievals of the same item from a database.

Example:
The first time we fetch the user with ID 1, we query the database and the User object is stored in the IdentityMap. Every subsequent call to fetch this same user will fetch it directly from the IdentityMap, which stores its objects in a Map.

[vartanbeno]

Closed, see #24.