Create PartParams class to allow multipart posts with JSON content an…

…d file upload

lib/faraday/autoload.rb

@@ -76,6 +76,7 @@ class Request
     extend AutoloadHelper
     autoload_all 'faraday/request',
                  UrlEncoded: 'url_encoded',
+                 PartParams: 'part_params',
                  Multipart: 'multipart',
                  Retry: 'retry',
                  Authorization: 'authorization',

lib/faraday/request/multipart.rb

@@ -52,7 +52,7 @@ def has_multipart?(obj) # rubocop:disable Naming/PredicateName
       def create_multipart(env, params)
         boundary = env.request.boundary
         parts = process_params(params) do |key, value|
-          Faraday::Parts::Part.new(boundary, key, value)
+          part(boundary, key, value)
         end
         parts << Faraday::Parts::EpiloguePart.new(boundary)
 
@@ -61,6 +61,14 @@ def create_multipart(env, params)
         body
       end
 
+      def part(boundary, key, value)
+        if value.is_a?(Faraday::Request::PartParams)
+          value.to_part(boundary, key)
+        else
+          Faraday::Parts::Part.new(boundary, key, value)
+        end
+      end
+
       # @return [String]
       def unique_boundary
         "#{DEFAULT_BOUNDARY_PREFIX}-#{SecureRandom.hex}"

lib/faraday/request/part_params.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module Faraday
+  class Request
+    # Allows multipart posts while specifying headers of a part
+    class PartParams
+      def initialize(value, headers)
+        @value = value
+        @headers = headers
+      end
+
+      def to_part(boundary, key)
+        Faraday::Parts::Part.new(boundary, key, value, headers)
+      end
+
+      attr_reader :value, :headers
+    end
+  end
+end

spec/faraday/request/multipart_spec.rb

@@ -68,6 +68,90 @@
     end
   end
 
+  context 'when providing json and IO content in the same payload' do
+    let(:io_content) { StringIO.new('io-content') }
+    let(:json_content) do
+      {
+        b: 1,
+        c: 2
+      }.to_json
+    end
+
+    let(:payload) do
+      {
+        json_content: Faraday::Request::PartParams.new(
+          json_content,
+          'Content-Type' => 'application/json'
+        ),
+        io_content: Faraday::UploadIO.new(io_content, 'application/pdf')
+      }
+    end
+
+    it_behaves_like 'a multipart request'
+
+    it 'forms a multipart request' do
+      response = conn.post('/echo', payload)
+
+      boundary = parse_multipart_boundary(response.headers['Content-Type'])
+      result = parse_multipart(boundary, response.body)
+      expect(result[:errors]).to be_empty
+
+      part_json, body_json = result.part('json_content')
+      expect(part_json).to_not be_nil
+      expect(part_json.mime).to eq('application/json')
+      expect(part_json.filename).to be_nil
+      expect(body_json).to eq(json_content)
+
+      part_io, body_io = result.part('io_content')
+      expect(part_io).to_not be_nil
+      expect(part_io.mime).to eq('application/pdf')
+      expect(part_io.filename).to eq('local.path')
+      expect(body_io).to eq(io_content.string)
+    end
+  end
+
+  context 'when providing json and IO content in the same payload' do
+    let(:io_content) { StringIO.new('io-content') }
+    let(:json_content) do
+      {
+        b: 1,
+        c: 2
+      }.to_json
+    end
+
+    let(:payload) do
+      {
+        json_content: Faraday::Request::PartParams.new(
+          json_content,
+          'Content-Type' => 'application/json'
+        ),
+        io_content: Faraday::UploadIO.new(io_content, 'application/pdf')
+      }
+    end
+
+    it_behaves_like 'a multipart request'
+
+    it 'forms a multipart request' do
+      response = conn.post('/echo', payload)
+
+      boundary = parse_multipart_boundary(response.headers['Content-Type'])
+      result = parse_multipart(boundary, response.body)
+      expect(result[:errors]).to be_empty
+
+      part_json, body_json = result.part('json_content')
+      expect(part_json).to_not be_nil
+      expect(part_json.mime).to eq('application/json')
+      expect(part_json.filename).to be_nil
+      expect(body_json).to eq(json_content)
+
+      part_io, body_io = result.part('io_content')
+      expect(part_io).to_not be_nil
+      expect(part_io.mime).to eq('application/pdf')
+      expect(part_io.filename).to eq('local.path')
+      expect(body_io).to eq(io_content.string)
+    end
+  end
+
   context 'when multipart objects in array param' do
     let(:payload) do
       {