Allow dynamic meta content about jsonapi-serializer

[SeyZ]

Hey @allistercsmith @bradbumbalough @mrkvon @sazzer

I've added the possibility to set a function for the meta key:
https://github.com/SeyZ/jsonapi-serializer/blob/master/test/serializer.js#L198

If you need a dynamic meta for a relationship, it was already implemented:
https://github.com/SeyZ/jsonapi-serializer/blob/master/test/serializer.js#L1549

I think it's fixed this issue, right? @bradbumbalough Hope you will come back here 👍 💃

[SeyZ]

Hey @allistercsmith ! Thanks for your message, I appreciate!

Good idea for the dynamic meta. You're totally right, it's not good to recreate a serializer each time the meta change.

I like the idea to pass an opts to the serialize method too. You should use the lodash merge method to merge the objects opts.meta and that.opts.meta :)

PR is of course welcomed!

[sazzer]

Once we're passing options in to the serialize function, it could be useful to make them available to all of the various functions that are declared as part of the schemas as well. That way, we can - for example - make the request details available inside the functions that are used to generate URLs, instead of having to solely depend on the record data or else re-creating the schema every time.

[mrkvon]

The first (and possibly sufficient) step, as suggested in #67 could be: Allow the meta in Serializer's opts to be a function (or object of functions if one wishes to be more strict).

// meta as a function
// fully dynamic meta
var UserSerializer = new JSONAPISerializer('users', {
  attributes: ['firstName', 'lastName'],
  meta: (data) => { // or meta: (data) => data.meta
    foo: 'bar',
    baz: data.baz
  }
});

// meta as an object of functions (optionally)
// which is more strict
// and similar to topLevelLinks and other ...Links options
var UserSerializer = new JSONAPISerializer('users', {
  attributes: ['firstName', 'lastName'],
  meta: {
    foo: 'bar', // static meta property
    baz: (data) => data.baz // dynamic meta property
  }
});

var users = UserSerializer.serialize(data);

This would be the most intuitive solution imho.

The implementation of 2nd option may be identical to the way topLevelLinks are done:

// lib/serializer.js
    function getMeta(meta) {
      return _.mapValues(meta, function (value) {
        if (_.isFunction(value)) {
          return value(records);
        } else {
          return value;
        }
      });
    }
...
    if (that.opts.meta) {
      payload.meta = getMeta(that.opts.meta);
    }

[bradbyte]

This was a requirement for my use case. Ended up switching to https://github.com/kutlerskaggs/json-api-ify as they provide a somewhat dynamic meta option.