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Bounty Ended with Falcon's answer chosen by SiberianGuy
Tweeted twitter.com/#!/StackProgrammer/status/103143995986808832
Bounty Started worth 100 reputation by SiberianGuy
removed abbreviation from title and start of post so people actually know what DDD is
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Jarrod Nettles
Jarrod Nettles
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DDD Domain-Driven-Design - external dependencies in the Entity problem

I'd like to start DDDDomain-Driven-Design, but there are several problems I'd like to solve before starting :)

Let's imagine I have a Groups and Users and when user wants to join a group, I'm calling groupsService.AddUserToGroup(group, user) method. In DDD I should do group.JoinUser(user), which looks pretty good.

The problem appears if I there are some validation rules for adding a user, or some external tasks need to be started when user is added to the group. Having these tasks will lead to the entity having external dependencies.

An example could be - a restriction that user can only participate in 3 groups max. This will require DB-calls from inside group.JoinUser method to validate this.

But the fact that an Entity depends on some external services/classes doesn't seem so good and "natural" to me.

What's the proper way to deal with this in DDD?

DDD - external dependencies in the Entity problem

I'd like to start DDD, but there are several problems I'd like to solve before starting :)

Let's imagine I have a Groups and Users and when user wants to join a group, I'm calling groupsService.AddUserToGroup(group, user) method. In DDD I should do group.JoinUser(user), which looks pretty good.

The problem appears if I there are some validation rules for adding a user, or some external tasks need to be started when user is added to the group. Having these tasks will lead to the entity having external dependencies.

An example could be - a restriction that user can only participate in 3 groups max. This will require DB-calls from inside group.JoinUser method to validate this.

But the fact that an Entity depends on some external services/classes doesn't seem so good and "natural" to me.

What's the proper way to deal with this in DDD?

Domain-Driven-Design - external dependencies in the Entity problem

I'd like to start Domain-Driven-Design, but there are several problems I'd like to solve before starting :)

Let's imagine I have a Groups and Users and when user wants to join a group, I'm calling groupsService.AddUserToGroup(group, user) method. In DDD I should do group.JoinUser(user), which looks pretty good.

The problem appears if I there are some validation rules for adding a user, or some external tasks need to be started when user is added to the group. Having these tasks will lead to the entity having external dependencies.

An example could be - a restriction that user can only participate in 3 groups max. This will require DB-calls from inside group.JoinUser method to validate this.

But the fact that an Entity depends on some external services/classes doesn't seem so good and "natural" to me.

What's the proper way to deal with this in DDD?

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Shaddix
Shaddix
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DDD - external dependencies in the Entity problem

I'd like to start DDD, but there are several problems I'd like to solve before starting :)

Let's imagine I have a Groups and Users and when user wants to join a group, I'm calling groupsService.AddUserToGroup(group, user) method. In DDD I should do group.JoinUser(user), which looks pretty good.

The problem appears if I there are some validation rules for adding a user, or some external tasks need to be started when user is added to the group. Having these tasks will lead to the entity having external dependencies.

An example could be - a restriction that user can only participate in 3 groups max. This will require DB-calls from inside group.JoinUser method to validate this.

But the fact that an Entity depends on some external services/classes doesn't seem so good and "natural" to me.

What's the proper way to deal with this in DDD?