Cayenne User Documentation
Relationships

Relationships are special DataObject properties that reference other "related" objects. Semantically there are two types of relationships - to-one pointing to just a single DataObjects (e.g. Painting.toArtist), and to-many pointing to a collection of DataObjects of the same base type (e.g. Artist.paintingArray).

### To-One Relationships

"Get" methods for to-one relationships return the target DataObject. If the object is not in memory yet, it will be fetched on demand. Modifying to-one relationships is no different from modifying attributes - just a simple call to a "set" method:

    Painting painting;

    // obtain artist for a given painting
    Artist originalArtist = painting.getToArtist();

    // replace with a new artist
    Artist newArtist = (Artist)context.createAndRegisterNewObject(Artist.class);
    painting.setToArtist(newArtist);

    // or remove Artist at all...
    // painting.setToArtist(null);

| When adding or removing an object from any kind of relationship, Cayenne will locate and modify an existing reverse relationship as appropriate.

### To-Many Relationships

"Get" methods for to-many relationships return Lists of DataObjects. Just like individual DataObjects, such lists are also resolved on demand (e.g. when a user tries to read an element from the list). For modification there are special "addTo..." and "removeFrom..." methods:

    Artist artist;

    // obtain a list of paintings
    List paintings = artist.getPaintingArray();

    // remove the first painting
    if(paintings.size() > 0) {
       Painting firstPainting = (Painting)paintings.get(0);
       artist.removeFromPaintingArray(firstPainting);
    }

    // add a new painting
    Painting newPainting = (Painting)context.createAndRegisterNewObject(Painting.class);
    artist.addToPaintingArray(newPainting);

| While to-many relationships in Cayenne are represented by Lists, they are really ordered Sets, as they are not allowed to contain the same object more than once.

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