A projection is the glue between an event stream and an entity.
The projection receives events as inputs, and modifies the state of an entity based on which event is being processed, and the content of the event.
When an entity is "retrieved", events from its event stream are applied to it by the entity projection.
# Example Projection
class Projection include EntityProjection entity_name :account apply Deposited do |deposited| account.id = deposited.account_id amount = deposited.amount account.deposit(amount) end apply Withdrawn do |withdrawn| account.id = withdrawn.account_id amount = withdrawn.amount account.withdraw(amount) end end