Can I put domain services inside domain entities?

Question

Say I have a domain entity User:

class User {
  constructor (username, email, password) {
    this.username = username
    this.email = email
    this.password = password
  }
}

Now, all users need a way to validate their username and email

There are two ways I can think of doing so:

1. Inside the domain entity (domain/entities/User.js)

class User {
  constructor (username, email, password) {
    this.username = username
    this.email = email
    this.password = password
  }
  
  validate () {
    // validation logic
    // ...

    if (!this.username.isValid || !this.email.isValid) {
      return false
    } else {
      return true
    }
  }
}

2. In a domain service (/domain/services/validateUser.js)

function validateUser (userToValidate) {
  // validation logic
  // ..
  
  if (userIsValid) {
    return true
  } else {
    return false 
  }
}

Either way, the use case/application service calls the validation logic

function createUserUseCase (username, email, password) {
  const user = new User(username, email, password)
  const isValid = user.validate()

  // ...
  // or... calling a service

  const user = new User(username, email, password)
  const isValid = validateUser(user)

}

So, should entities also carry logic or only hold data? Can I put validation logic inside an Entity, or do I have to create a service that receives a user and validates the user?

Thanks.

Answer 1

Answering your question (about services)

A domain service is by definition always outside of a domain entity:

SERVICE: An operation offered as an interface that stands alone in the model, with no encapsulated state.
- Eric Evans

A service is meant for operations that do not belong to the entity, for example because it's not a responsibility of the entity and should therefore be decoupled from it, or because it's about several entities.

So by definition you cannot have a service "INSIDE" an entity. "Inside" the entitiy, it's called an operation. This formally answers your question.

Answering the question behind your question (where to validate)

Unfortunately this doesn't tell you where to put the validation: should it be an operation of the entity? Or shall it be in a service? There are many factors to consider, and there are numerous questions on SE (for example this recent one or this older one) on where to put a validator.

Both are possible, but it really depends on how you see the world in your design. The following questions shall help you to make your own mind with solid arguments:

• is the validation a responsibility of the entity ?
• is there any behavioral difference between a validated entity and a non-validated entity? (i.e. not the behaviors of the context that uses the User, but really of User itself) ?
• may the validation change for other reasons than a change in the entity? And in particular, could the validation in principle involve other entities / or even repositories (e.g.verifying that no other user exists with the same email, or that the domain of the email is not on a blacklist).
• could there be reasons that justify that validation shall be performed before the User is even created, i.e. the validation is clearly not a responsibility of User?
• is it possible that there are different kind of validation with different purposes? E.g. a part being the responsibility of the entity, a part being of the responsibility of a service?

Now you can assess yourself, if it's a good idea or not to have the validation INSIDE as operation, or OUTSIDE as service.