allows sparse fieldsets for multiple resources

README.md

@@ -213,6 +213,10 @@ article.title
 # should not have returned the created_at
 article.created_at
 # => raise NoMethodError
+
+# or you can use fieldsets from multiple resources
+# makes request to /articles?fields[articles]=title,body&fields[comments]=tag
+article = Article.select("title", "body",{comments: 'tag'}).first
 ```
 
 ## Sorting
@@ -485,7 +489,7 @@ class MyMoneyCaster
     end
   end
 end
-   
+
 JsonApiClient::Schema.register money: MyMoneyCaster
 
 ```

lib/json_api_client/query/builder.rb

@@ -35,11 +35,7 @@ def includes(*tables)
       end
 
       def select(*fields)
-        fields = Array(fields).flatten
-        fields = fields.map { |i| i.to_s.split(",") }.flatten
-
-        @fields += fields.map(&:strip)
-
+        @fields += parse_fields(*fields)
         self
       end
 
@@ -143,7 +139,23 @@ def order_params
       end
 
       def select_params
-        @fields.empty? ? {} : {fields: {klass.table_name => @fields.join(",")}}
+        if @fields.empty?
+          {}
+        else
+          field_result = Hash.new { |h,k| h[k] = [] }
+          @fields.each do |field|
+            if field.is_a? Hash
+              field.each do |k,v|
+                field_result[k.to_s] << v
+                field_result[k.to_s] = field_result[k.to_s].flatten
+              end
+            else
+              field_result[klass.table_name] << field
+            end
+          end
+          field_result.each { |k,v| field_result[k] = v.join(',') }
+          {fields: field_result}
+        end
       end
 
       def parse_related_links(*tables)
@@ -179,6 +191,21 @@ def parse_orders(*args)
         end.flatten
       end
 
+      def parse_fields(*fields)
+        fields = fields.split(',') if fields.is_a? String
+        fields.map do |field|
+          case field
+          when Hash
+            field.each do |k,v|
+              field[k] = parse_fields(v)
+            end
+            field
+          else
+            Array(field).flatten.map { |i| i.to_s.split(",") }.flatten.map(&:strip)
+          end
+        end.flatten
+      end
+
     end
   end
 end

test/unit/query_builder_test.rb

@@ -132,4 +132,41 @@ def test_can_select_fields_using_implicit_array
     Article.select(:title, :body).to_a
   end
 
+  def test_can_select_nested_fields_using_hashes
+    stub_request(:get, "http://example.com/articles")
+      .with(query: {fields: {articles: 'tags', comments: 'author'}})
+      .to_return(headers: {content_type: "application/vnd.api+json"}, body: {
+        data: []
+      }.to_json)
+    Article.select({comments: :author}, :tags).to_a
+  end
+
+
+  def test_can_select_nested_fields_using_hashes_of_arrays
+    stub_request(:get, "http://example.com/articles")
+      .with(query: {fields: {articles: 'tags', comments: 'author,text'}})
+      .to_return(headers: {content_type: "application/vnd.api+json"}, body: {
+        data: []
+      }.to_json)
+    Article.select({comments: [:author, :text]}, :tags).to_a
+  end
+
+  def test_can_select_nested_fields_using_strings
+    stub_request(:get, "http://example.com/articles")
+      .with(query: {fields: {articles: 'tags', comments: 'author,text'}})
+      .to_return(headers: {content_type: "application/vnd.api+json"}, body: {
+        data: []
+      }.to_json)
+    Article.select({comments: ['author', 'text']}, :tags).to_a
+  end
+
+  def test_can_select_nested_fields_using_comma_separated_strings
+    stub_request(:get, "http://example.com/articles")
+      .with(query: {fields: {articles: 'tags', comments: 'author,text'}})
+      .to_return(headers: {content_type: "application/vnd.api+json"}, body: {
+        data: []
+      }.to_json)
+    Article.select({comments: 'author,text'}, :tags).to_a
+  end
+
 end