Grails many to many relationship migration - grails

Question

Hello I have two domain classes as following
class Users {
String password
String firstName
String lastName
String emailAddress
String username
Company company
.....
static hasMany = [projects:Projects];
}
Another class
class Projects {
String projectName
String description
Users projectLead
Date dateCreated
Date lastUpdated
static belongsTo = Users
}
These classes obviously has one to many relationship but now I want to change it to many to many relationship by adding "ProjectMembership" class but the problem I have is that my application has already gone into production and there are people who are already using the app. In such a case they already have one user->many projects in the the db. In such a case how can I migrate this existing data and change my prod app to have m2m relationship which will looks like following.
class Users {
String password
String firstName
String lastName
String emailAddress
String username
Company company
.....
static hasMany = [projectMemberships:ProjectMemberships];
}
Another class
class Projects {
String projectName
String description
Users projectLead
Date dateCreated
Date lastUpdated
static hasMany = [projectMemberships:ProjectMemberships];
}
and
class ProjectMemberships{
Users u
Projects p
}

Answer 1

This is best done with a migration tool like Liquibase, and the http://grails.org/plugin/database-migration plugin is probably your best be in Grails since it uses Liquibase and is tightly integrated with GORM. But this one's easy enough to do by hand.
I wouldn't use hasMany since you can easily manage everything from the ProjectMemberships class, so your Users and Projects classes would be
class Users {
String password
String firstName
String lastName
String emailAddress
String username
Company company
.....
}
and
class Projects {
String projectName
String description
Date dateCreated
Date lastUpdated
}
I'd go with a ProjectMemberships class that uses a composite key, which requires that it implement Serializable and have a good hashCode and equals:
import org.apache.commons.lang.builder.HashCodeBuilder
class ProjectMemberships implements Serializable {
Users u
Projects p
boolean equals(other) {
if (!(other instanceof ProjectMemberships)) {
return false
}
other.u?.id == u?.id && other.p?.id == p?.id
}
int hashCode() {
def builder = new HashCodeBuilder()
if (u) builder.append(u.id)
if (p) builder.append(p.id)
builder.toHashCode()
}
static ProjectMemberships get(long userId, long projectId) {
find 'from ProjectMemberships where u.id=:userId and p.id=:projectId',
[userId: userId, projectId: projectId]
}
static ProjectMemberships create(Users u, Projects p, boolean flush = false) {
new ProjectMemberships(u: u, p: p).save(flush: flush, insert: true)
}
static boolean remove(Users u, Projects p, boolean flush = false) {
ProjectMemberships instance = ProjectMemberships.findByUsersAndProjects(u, p)
if (!instance) {
return false
}
instance.delete(flush: flush)
true
}
static void removeAll(Users u) {
executeUpdate 'DELETE FROM ProjectMemberships WHERE u=:u', [u: u]
}
static void removeAll(Projects p) {
executeUpdate 'DELETE FROM ProjectMemberships WHERE p=:p', [p: p]
}
static mapping = {
id composite: ['p', 'u']
version false
}
}
Use ProjectMemberships.create() to add a relationship between a user and a project, and ProjectMemberships.remove() to remove it.
Run grails schema-export to see the updated DDL (it'll be in target/ddl.sql). Run the create table statement for the project_memberships table, e.g.
create table project_memberships (
p_id bigint not null,
u_id bigint not null,
primary key (p_id, u_id)
)
Then populate it with this SQL (depending on your database you might need a slightly different syntax):
insert into project_memberships(p_id, u_id) select id, project_lead_id from projects
and finally drop the project_lead_id column from the projects table.
Of course do a database backup before making any changes.
You can get a user's projects with
def projects = ProjectMemberships.findAllByUsers(user)*.p
and similarly a project's users with
def users = ProjectMemberships.findAllByProjects(project)*.u