Links on a relationship can take a callable

README.md

@@ -305,6 +305,17 @@ This will create a `self` reference for the relationship, and a `related` link f
   }
 ```
 
+Relationship links can also be configured to be defined as a callable.
+
+```ruby
+  has_many :actors, links: -> (object, params) {
+    {
+      self: "https://movies.com/#{object.id}/relationships/actors",
+      next: "https://movies.com/#{object.id}/relationships/actors?page%5Bnumber%5D=2&page%5Bsize%5D=10"
+    }
+  }
+```
+
 ### Meta Per Resource
 
 For every resource in the collection, you can include a meta object containing non-standard meta-information about a resource that can not be represented as an attribute or relationship.

lib/fast_jsonapi/relationship.rb

@@ -148,6 +148,8 @@ module FastJsonapi
     def add_links_hash(record, params, output_hash)
       output_hash[key][:links] = if links.is_a?(Symbol)
                                    record.public_send(links)
+                                 elsif links.respond_to?(:call)
+                                   FastJsonapi.call_proc(links, record, params)
                                  else
                                    links.each_with_object({}) do |(key, method), hash|
                                      Link.new(key: key, method: method).serialize(record, params, hash)

spec/fixtures/movie.rb

@@ -124,3 +124,15 @@ module Cached
     end
   end
 end
+
+class CallableLinksMovieSerializer < ::MovieSerializer
+  has_many(
+    :first_two_actors,
+    id_method_name: :uid,
+    links: lambda do |record, params|
+      { some: record.id, fancy: 'here' }
+    end
+  ) do |record, params|
+    record.actors.take(2)
+  end
+end

spec/integration/relationships_spec.rb

@@ -142,5 +142,18 @@ RSpec.describe JSONAPI::Serializer do
         end
       end
     end
+
+    context 'with a callable as relationship links' do
+      let(:serialized) do
+        CallableLinksMovieSerializer.new(movie, params).serializable_hash.as_json
+      end
+
+      it do
+        expect(serialized['data']['relationships']['first_two_actors'])
+          .to have_link('some').with_value(movie.id)
+        expect(serialized['data']['relationships']['first_two_actors'])
+          .to have_link('fancy').with_value('here')
+      end
+    end
   end
 end