HasAndBelongsToMany relations

</figure>

For example, here is an excerpt from a model JSON file for an assembly model, expressing a hasAndBelongsToMany relation between assembly and part models:

{
  "name": "Assembly",
  "plural": "Assemblies",
  "relations": {
    "parts": {
      "type": "hasAndBelongsToMany",
      "model": "Part",
      "throughTable": "AssemblyPart"
    },
...

If needed, you can use the “throughTable” field to define a custom table name for the relation.

You can also define a hasAndBelongsToMany relation in code, though this is not recommended in general. For example:

Part.hasAndBelongsToMany(Assembly);
Assembly.hasAndBelongsToMany(Part);

Adding a relation via REST API

When adding relation through the REST API, a join model must exist before adding relations. For Example in the above example with “Assembly” and “Part” models, to add an instance of “Part” to “Assembly” through the REST API interface an “AssemblyPart” model must exist for it to work.

Most of the time you should add “hasAndBelongToMany” relations to models on server side using the method:

assembly.parts.add(part, function(err) {
  //...
});

Thus, if you need to add the relation using REST, first check if the “AssemblyPart” model exists first. Then add the relation using this code:

Assembly.Parts.link({id:assemblyId, fk: partId}, partInstance,  function(value, header) {
  //success
});

Methods added to the model

Once you define a “hasAndBelongsToMany” relation, LoopBack adds methods with the relation name to the declaring model class’s prototype automatically. For example: assembly.parts.create(...).

assembly.parts(filter,
  function(err, parts) {
  ...
});

var part = assembly.parts.build(data);

assembly.parts.create(data,
  function(err, part) {
  ...
});

assembly.parts.add(part,
  function(err) {
  ...
});

assembly.parts.remove(part,
  function(err) {
  ...
});

assembly.parts.findById(partId,
  function(err, part) {
  ...
});

assembly.parts.destroy(partId,
function(err) {
  ...
});