Introduce Relationships

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• A server MUST support fetching relationship data for every relationship URL provided as a self link as part of a relationship’s links object.

GET /articles/1/relationships/author HTTP/1.1
Accept: application/vnd.api+json

Responses

200 OK

• A server MUST respond to a successful request to fetch a relationship with a 200 OK response.
• The primary data in the response document MUST match the appropriate value for resource linkage, as described above for relationship objects.
• The top-level links object MAY contain self and related links, as described above for relationship objects.
• A one to one relationship returns the object.
• One to many an array of objects.

HTTP/1.1 200 OK
Content-Type: application/vnd.api+json

{
  "links": {
    "self": "/articles/1/relationships/tags",
    "related": "/articles/1/tags"
  },
  "data": [
    { "type": "tags", "id": "2" },
    { "type": "tags", "id": "3" }
  ]
}

• No data returns an empty array

404 Not Found

• A server MUST return 404 Not Found when processing a request to fetch a relationship link URL that does not exist. Note: This can happen when the parent resource of the relationship does not exist. For example, when /articles/1 does not exist, request to /articles/1/relationships/tags returns 404 Not Found.

• If a relationship link URL exists but the relationship is empty, then 200 OK MUST be returned, as described above.

• A server MAY respond with other HTTP status codes.

• A server MAY include error details with error responses.

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Resolved in #43