Issue with many-to-many and mapping

This is my first project with many-to-many relationship and after reading some grails stuff of ORM, I decided to let grails do the work for me, like creating the tables. Actually, my deal was:

• 1 User has many Groups
• 1 Group belongs to an User and has many Users(except the owner)

Here are my classes:

class User {  
    String name
    List groups

    static hasMany = [groups : Group]

    static mapping = {
        table 'users'
        version false
    }
}

class Group {   
    String name
    List members

    static belongsTo = User
    static hasMany = [members : User]

    static mapping = {
        table 'groups'
        version false
    }
}

After that, what I got from grails was 3 tables. The users table was ok, but the next 2 wasn't what I expected.

table **groups** |  id  |  name
table **users_groups** |  group_id  |  user_id  |  members_idx  |  groups_idx

Well, what I wanted was something like this:

table **groups** |  id  |  name  |   user_id 
table **users_groups** |  group_id  |  members_idx  |  groups_idx

I don't know if I did something wr开发者_StackOverflowong or how can I fix it, but anyway... I'm still getting another problem when I try to do user.addToGroups(new Group()) . My users_groups table duplicates this register, one with the members_idx null, and the other register with the groups_idx null.

How about removing belongsTo from group and leave hasMany:

class User {  
    String name
    List groups

    static hasMany = [groups : Group]

    static mapping = {
        table 'users'
        version false
    }
}

class Group {   
    String name
    List members

    static hasMany = [members : User]

    static mapping = {
        table 'groups'
        version false
    }
}

EDIT I think You have to make a domain class which will actually join groups and users. I think You should try this:

    class User {  
        String name

        static hasMany = [groups : UsersGroups]

        static mapping = {
            table 'users'
            version false
        }
    }

class Group {   
    String name

    static hasMany = [members : UsersGroups]

    static mapping = {
        table 'groups'
        version false
    }
}

class UsersGroups {

   static belongsTo = [group: Group, user: User]
   static mapping = {
      version false
   }

}

It will create another table which actually joins groups and users.

If you change the belongsTo to use Map syntax it will use a foreign key and not a join table:

static belongsTo = [user: User]

and you'll have a user field as a back-reference.

I have done a similar issue before, so I think this may work:

class User {  
    String name
    hasMany = [groups : Group]

    static mapping = {
        table 'users'
        version false
    }
}

class Group {   
    String name

    static belongsTo = [owner : User]
    static hasMany = [members : User]

    static mapping = {
        table 'groups'
        version false
    }
}

About the problem with user.addToGroups, I think that you should set the owner when creating the group first. Then add other users to group.