I am new to event sourcing, but as fas as I have understood when we have a command use case, we instantiate an aggregate in memory, apply events to it from the event store so as to be in the correct state, make the proper changes and then store those changes back to the event store. We also have a read model store that will eventually be updated by these changes.
In my case I have a CreateUserUseCase (which is a command use case) and I want to first check if the user already exists and if the username is already taken. For example something like this:
const userAlreadyExists = await this.userRepo.exists(email);
if (userAlreadyExists) {
return new EmailAlreadyExistsError(email);
}
const alreadyCreatedUserByUserName = await this.userRepo
.getUserByUserName(username);
if (alreadyCreatedUserByUserName) {
return new UsernameTakenError(username);
}
const user = new User(username, password, email);
await this.userRepo.save(user);
So, for the save method I would use the event store and append the uncommitted events to it. What about the exists and getUserByUserName methods though? On the one hand I want to make a specific query so I could use my read model store to get the data that I need, but on the other hand this makes a contradiction with CQRS. So what do we do in these cases? Do we, in some way, perform queries to the event store? And in what way do we do this?
Thank you in advance!