CODEWITHSUNDEEP
grailsgrails-orm
Anthony
Anthony

Reputation: 35928

Understanding hasMany behavior in grails and how to save a relationship

I'm making a model where a User fills out many questionnaires and the responses get saved to Questionresponse. I'm on grails 2.5.2

Test1

So I have two models

class User {
  String username
  ...
  static hasMany = [reponse: QuestionResponse]
}

class QuestionResponse {
  String question_1
  String question_2
  ...
}

With the above, a new DB table is created: user_questionresponse with two columns user_questionresponses_id and questionresponse_id. This seems like what I want. A user would have many questionresponses and those relationships would be saved in this table. However, I can't find out how to save data to this table.

For example, if I do:

def user = springSecurityService.currentUser
def questionnaire = new QuestionResponse(question_1: "foo", question_2: "bar")
//How do I link the user to this newly created questionnaire?
user.addToResponse(q).save(flush: true) //DOES NOT WORK.

Test2 (just add belongsTo)

class User {
  String username
  ...
  static hasMany = [reponse: QuestionResponse]
}

class QuestionResponse {
  String question_1
  String question_2
  static belongsTo = [user: User]
  ...
}

If I add belongsTo to QuestionResponse a new column, user_id, gets created in the DB. Now if I run the same code as above, this user_id column has the id populated with that of the current user. However, the relationship table, user_questionresponse is still empty.

I am aware of the approach mentioned by Burt but I assume that should be required only for ManyToMany relationship. If that is required for all relationship, why isn't that the default?

Upvotes: 3

Views: 1931

Answers (2)

ncerezo
ncerezo

Reputation: 1371

The second approach (adding belongsTo in QuestionResponse) seems the right thing to do in your case, since QuestionResponse objects cannot exists without a user and cannot belong to different users.

In that case there's no need to use a third table.

When you run the app for the first time, grails created the relation table (because there was no belongsTo). When you run the app again with belongsTo grails adds the user_id field but DOES NOT DROP the relation table. That's why the table is there and is empty: it's not needed, but grails database auto-update feature only adds things, it does not remove anything.

The same applies to fields: if you remove a field from an entity you have to manually remove it from the database.

Upvotes: 1

Sandeep Poonia
Sandeep Poonia

Reputation: 2188

In your first case, you have a OneToMany relationship between User and QuestionResponse with no side being the owner of the relationship. In this case to maintain the relationship between User and QuestionResponse, a third table is required. To persist data you need to do the:

userInstance.addToResponse(new QuestionResponse(question_1: "foo", question_2: "bar")).save(flush: true, failOnError: true)

You are doing user.addToReponse(q) instead it should be user.addToReponse(questionnaire), if it's not a typo and the data is actually not being stored, then check by adding the failOnError parameter to save() method. Sometimes grails save() method fails silently, it should tell you if this is the case.

In second case, you have added the parent to the relationship, so that means you don't need the third table to maintain the relationship. Grails will not create and populate the third table in this case.

Upvotes: 2

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