Grails: How to query objects in many to many mapping?

问题

Hello I have the follwing domain classes.

class Student {
   int age
   static hasMany = [courses:Course]
}

class Course {
   String name
   static hasMany = [students:Student]
}

I want to find the Students taking Course (with id 1), with age 7.

Could I do that with dynamic finder or criteria builder or HQL?

I do not want to do following as it load all students so inefficient:

def course = Course.get(1);
course.students.findAll{ it.age == 7 }

回答1:

def studs = Student.withCriteria {
  eq('age', 7)
  courses {
    eq('id', 1)
  }
}

It's in GORM doc, section "Querying Associations".

回答2:

You can use a dynamic finder:

def students = Student.findByCourseAndAge(Course.load(1), 7)

By using load() instead of get() you avoid retrieving the whole Course instance and just reference its id.

Another option is HQL:

def students = Student.executeQuery(
   'from Student s where s.course.id = :courseId and s.age = :age',
   [courseId: 1, age: 7])

来源：https://stackoverflow.com/questions/6316761/grails-how-to-query-objects-in-many-to-many-mapping