Grails query association problem

Question

I'm having trouble writing a query for the following domain classes:

class Person {
  static hasMany = [memberships: Membership]
}

class Membership {

    static belongsTo = [person: Person, group: Group]

    Date joinDate = new Date();
    Group group;
    Person person;
}

class Group {
     static hasMany = [memberships: Membership]
}

Basically, I want to found all persons which belongs to a list of groups (Let's say group ids are (1,2). The trick here is that the person must be a member of both groups. I'd prefer a criteria query, but HQL is ok also.

Note that querying with something like group.id in (1,2) won't work because it can be any of the groups, not both.

Answer 1

That's my simple HQL approach:

Person.executeQuery("FROM Person x WHERE x IN (SELECT m.person from Membership m WHERE m.group = :group1) AND x IN (SELECT m.person from Membership m WHERE m.group = :group2)", [ group1: Group.get(1), group2: Group.get(2) ])

Cheers

Answer 2

Maybe you don't need a query. In the Person class, memberships is a list of Membership objects. You can find if an object is in a Collection (list) without making any query. Something like this should do the job.

if(Person.memberships.contains(Membership.findByPersonAndGroup(person1,group1)) && Person.memberships.contains(Membership.findByPersonAndGroup(person1,group2))){
  ...do something...
}

Probably the other solution is simplier, but I think this is just another option.

Cheers

Groovy Collections

Answer 3

Interesting problem. Not sure the previous solutions are generic on the number of matched groups - in the cases so far, it's fixed at 2 I think. Although there is probably a way to make them variable.

Another way I describe here on the grails messageboard - http://www.nabble.com/has-many-through-relationship-query---GORM--td23438096.html

Including commentary from the author, Robert Fischer, of "Grails Persistence with GORM and GSQL".

@chadsmall

Answer 4

Here's another approach that avoids having to programmatically append subqueries to your WHERE clause:

Query:

SELECT count(person.id) AS numPeople, person 
FROM Person as person 
INNER JOIN 
person.memberships AS mships
WITH mships.group.id IN (:groupIds) 
GROUP BY person.id 
HAVING COUNT(person.id) = (:numOfGroupIds)

And some example values:

[
  groupIds: [8,9,439,86843]
  numOfGroupIds: 4
]

The part of this query up to the GROUP BY grabs all of the people who match any of the groups. Then, by grouping by person and checking the number of results is equal to the number of groups in the list, you can verify that this person is a member of all of the specified groups.