Add support for polymorphic association in a resource

[pixelhandler]

• Update findRelated adapter method to be friendly to polymorphic associations
• Add example (in dummy app) of solution using polymorphic association for 'imageable' relation
• Add acceptance test for resource (pictures) that has polymorphic association (imageable)

The dummy app uses resources: employee, product and pictures. The pictures have an imageable relation which can be either an employee or product resource. There is no model prototype for the imageable resources only a service.

The solution to support polymorphic associations did require an imageable service, but this service only utilizes the findRelated method which uses the URLs provided in the relationship objects of the resource.

See the changed files list. Only a minor change in the adapter#findRelated method was needed for the polymorphic association support of a related resource. However the implementation does need a service that represents the polymorphic relation, in this case imageable is the service that supports types for employee or product.

Some links for the rails api:

• http://guides.rubyonrails.org/association_basics.html#polymorphic-associations
• https://github.com/cerebris/jsonapi-resources#options (used polymorphic option)
• Example API I used for the dummy app: https://github.com/pixelhandler/blog-api/compare/polymorphic-example

Below is a payload for a resource that uses polymorphic associations.

There are 3 types: Employee, Product, Picture. The Picture belongs to an Imageable resource (Employees and Products both use a has_many relation for Pictures).

In the payload below the request is for http://localhost:3000/api/v1/pictures?include=imageable so the 1st 5 of 6 pictures are returned and included are the imaginable types a few products and an employee.

The links under each picture resource's relationships objects have the url which should return the imageable resource regardless of type. E.g. http://localhost:3000/api/v1/pictures/5/imageable may be an Employee and http://localhost:3000/api/v1/pictures/4/imageable may be a Product.

{
  "data": [
    {
      "id": "1",
      "type": "pictures",
      "links": {
        "self": "http://localhost:3000/api/v1/pictures/1"
      },
      "attributes": {
        "name": "box of chocolates"
      },
      "relationships": {
        "imageable": {
          "links": {
            "self": "http://localhost:3000/api/v1/pictures/1/relationships/imageable",
            "related": "http://localhost:3000/api/v1/pictures/1/imageable"
          },
          "data": {
            "type": "products",
            "id": "3"
          }
        }
      }
    },
    {
      "id": "2",
      "type": "pictures",
      "links": {
        "self": "http://localhost:3000/api/v1/pictures/2"
      },
      "attributes": {
        "name": "10 foot candy cane"
      },
      "relationships": {
        "imageable": {
          "links": {
            "self": "http://localhost:3000/api/v1/pictures/2/relationships/imageable",
            "related": "http://localhost:3000/api/v1/pictures/2/imageable"
          },
          "data": {
            "type": "products",
            "id": "2"
          }
        }
      }
    },
    {
      "id": "3",
      "type": "pictures",
      "links": {
        "self": "http://localhost:3000/api/v1/pictures/3"
      },
      "attributes": {
        "name": "Hot apple fritter"
      },
      "relationships": {
        "imageable": {
          "links": {
            "self": "http://localhost:3000/api/v1/pictures/3/relationships/imageable",
            "related": "http://localhost:3000/api/v1/pictures/3/imageable"
          },
          "data": {
            "type": "products",
            "id": "1"
          }
        }
      }
    },
    {
      "id": "4",
      "type": "pictures",
      "links": {
        "self": "http://localhost:3000/api/v1/pictures/4"
      },
      "attributes": {
        "name": "Boston Creme"
      },
      "relationships": {
        "imageable": {
          "links": {
            "self": "http://localhost:3000/api/v1/pictures/4/relationships/imageable",
            "related": "http://localhost:3000/api/v1/pictures/4/imageable"
          },
          "data": {
            "type": "products",
            "id": "1"
          }
        }
      }
    },
    {
      "id": "5",
      "type": "pictures",
      "links": {
        "self": "http://localhost:3000/api/v1/pictures/5"
      },
      "attributes": {
        "name": "Bill at EmberConf"
      },
      "relationships": {
        "imageable": {
          "links": {
            "self": "http://localhost:3000/api/v1/pictures/5/relationships/imageable",
            "related": "http://localhost:3000/api/v1/pictures/5/imageable"
          },
          "data": {
            "type": "employees",
            "id": "1"
          }
        }
      }
    }
  ],
  "included": [
    {
      "id": "3",
      "type": "products",
      "links": {
        "self": "http://localhost:3000/api/v1/products/3"
      },
      "attributes": {
        "name": "Chocolates"
      },
      "relationships": {
        "pictures": {
          "links": {
            "self": "http://localhost:3000/api/v1/products/3/relationships/pictures",
            "related": "http://localhost:3000/api/v1/products/3/pictures"
          }
        }
      }
    },
    {
      "id": "2",
      "type": "products",
      "links": {
        "self": "http://localhost:3000/api/v1/products/2"
      },
      "attributes": {
        "name": "Candy Canes"
      },
      "relationships": {
        "pictures": {
          "links": {
            "self": "http://localhost:3000/api/v1/products/2/relationships/pictures",
            "related": "http://localhost:3000/api/v1/products/2/pictures"
          }
        }
      }
    },
    {
      "id": "1",
      "type": "products",
      "links": {
        "self": "http://localhost:3000/api/v1/products/1"
      },
      "attributes": {
        "name": "Donuts"
      },
      "relationships": {
        "pictures": {
          "links": {
            "self": "http://localhost:3000/api/v1/products/1/relationships/pictures",
            "related": "http://localhost:3000/api/v1/products/1/pictures"
          }
        }
      }
    },
    {
      "id": "1",
      "type": "employees",
      "links": {
        "self": "http://localhost:3000/api/v1/employees/1"
      },
      "attributes": {
        "name": "Bill Heaton"
      },
      "relationships": {
        "pictures": {
          "links": {
            "self": "http://localhost:3000/api/v1/employees/1/relationships/pictures",
            "related": "http://localhost:3000/api/v1/employees/1/pictures"
          }
        }
      }
    }
  ],
  "meta": {
    "page": {
      "total": 7,
      "sort": [
        {
          "field": "id",
          "direction": "asc"
        }
      ],
      "offset": 0,
      "limit": 5
    }
  },
  "links": {
    "first": "http://localhost:3000/api/v1/pictures?include=imageable&page%5Blimit%5D=5&page%5Boffset%5D=0",
    "next": "http://localhost:3000/api/v1/pictures?include=imageable&page%5Blimit%5D=5&page%5Boffset%5D=5",
    "last": "http://localhost:3000/api/v1/pictures?include=imageable&page%5Blimit%5D=5&page%5Boffset%5D=2"
  }
}

[oliverbarnes]

Looks good to me 👍

[pixelhandler]

@oliverbarnes thanks for taking a look. I need to add some more tests for the polymorphic association then will be ready to merge.