Question

I’ve two domain classes which are bonded with a bidirectional one-to-one relationship to each other, this being implemented using hasOne.

class AssessmentData {

    static hasOne = [assessmentField:AssessmentField, assessment:Assessment]

    AssessmentField field
}


class Assessment {
   AssessmentData assessmentData
}

But I’m quite a bit confused about the way Grails implements this relationship at the database level. As described here, it simply sets the foreign key at the child domain class only, in my case, to the Assessment and AssessmentField tables. Its my primitive instinct that both tables should have a foreign key referring to each other in order to establish a one-to-one bidirectional relationship. But since this is not the case, I wanna know how this is achieved by Grails.

Answer 1

Its my primitive instinct that both tables should have a foreign key referring to each other in order to establish a one-to-one bidirectional relationship

Your instincts have failed you. The domain described in your question will generate these 2 database tables:

assessment_data
----------------    
id


assessment
----------------
id
assessment_data_id

Using SQL we can get the assessment associated with an assessment_data with id 4

select * from assessment where assessment_data_id = 4

We can also go the other way and get the assessment_data associated with an assessment with id 5 using

select ad.* from assessment_data ad
inner join assessment a ON a.assessment_data_id = ad.id
where a.id = 5

So if we can “go both ways” in SQL, then we can do it with HQL, criteria queries, dynamic finders, etc. because these all ultimately get translated to SQL.