How to populate a ManyToMany relationship using YA

问题:

I have the following ManyToMany (bidirectional) relationship:

@Entity
public class Proposal extends Model {
    ...
    @ManyToMany
    public List<Tag> tags;
}

@Entity
public class Tag extends Model {
    ...
    @ManyToMany(mappedBy="tags")
    public List<Proposal> taggedProposals;
}

And I want to populate my DB with some test data using a yaml file (to display later using a simple view). This is part of my yaml file:

...
- &prop2 !!models.Proposal
    id:         2
    title:      Prop2 title
    proposer:   *user2

- &prop3 !!models.Proposal
    id:         3
    title:      Prop3 title
    proposer:   *user3

# Tags

- &tag1 !!models.Tag
    name: Tag1 name
    desc: Tag1 description
    taggedProposals:
        - *prop1

- &tag2 !!models.Tag
    name: Tag2 name
    desc: Tag2 description
    taggedProposals:
        - *prop2
        - *prop3

The problem is that when I try to display a Proposal's tags or a Tag's taggedProposals, the ArrayLists are empty! I tried using square brackets and commas without success. All the other data is being loaded and displayed correctly.

回答1:

If the answer posted by Leon Radley was accurate, it is no longer the case ! Play evolved and, since the 2.1 version, manyToMany reference initialization by list now work (see this link) ! See User.zones for an example of how it works.

zones:
    - &zone1 !!models.Zone
        id:                 1
        gtbName:            "ZZ01"
    - &zone2 !!models.Zone
        ...

users:
    - &user4 !!models.User
        id:                 4
        profile:            *profile4
        defaultZone:        *zone3
        zones:
            - *zone1
            - *zone2
            - *zone3

回答2:

The problem you have encountered happens because play uses ebean and ebean doesn't automagically saves many-to-many associations.

I had to solve it this way:

private static void initialData() {
    @SuppressWarnings("unchecked")
    Map<String,List<Object>> all = (Map<String,List<Object>>) Yaml.load("initial-data.yml");

    // Save all roles
    Ebean.save(all.get("roles"));

    // Insert users and for every user save its many-to-many association
    Ebean.save(all.get("users"));
    for(Object user: all.get("users")) {
        Ebean.saveManyToManyAssociations(user, "roles");
    }
}

And the yaml file:

# Roles
roles:
  - &adminRole !!models.Role
    name: admin

  - &projectleadRole !!models.Role
    name: projectlead

# Users
users:
  - &leonUser !!models.User
    email: [e-mail]
    roles:
     - *adminRole
     - *projectleadRole
    firstName: Leon
    lastName: Radley