Way to specify resource's fields list in RESTful API request

I'd go for option 2 IMHO.

So if the consumer just requests the resource url (/posts/42) they receive the default fields.

Then consumers can alter the default response by defining values in the query string like:

/posts/42/fields?subject,author_name

This has worked well for me in the past and is how some other well know APIs work, e.g. Facebook

Edit: Looking back at this I’d change the request to be:

/posts/42?fields=subject,author_name

/post/42 is the resource, not fields.

Have been doing research into this as well and was pointed towards Facebook's GraphQL as an alternative to requesting a restful api with the fields wanted. It is still in the very early stages but seems very promising.

https://facebook.github.io/react/blog/2015/05/01/graphql-introduction.html

EDIT: Reproduced from the URL:

A GraphQL query is a string interpreted by a server that returns data in a specified format. Here is an example query:

{
  user(id: 3500401) {
    id,
    name,
    isViewerFriend,
    profilePicture(size: 50)  {
      uri,
      width,
      height
    }
  }
}

(Note: this syntax is slightly different from previous GraphQL examples. We've recently been making improvements to the language.)

And here is the response to that query.

{
  "user" : {
    "id": 3500401,
    "name": "Jing Chen",
    "isViewerFriend": true,
    "profilePicture": {
      "uri": "http://someurl.cdn/pic.jpg",
      "width": 50,
      "height": 50
    }
  }
}