Polish "Add support for documenting request part payload fields"

- Rebase on latest code, and make use of new support for the same
  TemplatedSnippet producing multiple snippets with different names
  from the same template
- Expand the documentation
- Apply code formatting
- Add support for relaxed documentation of a request part's fields

Closes gh-270

docs/src/docs/asciidoc/documenting-your-api.adoc

@@ -415,7 +415,7 @@ include::{examples-dir}/com/example/mockmvc/RequestParameters.java[tags=request-
 ----
 <1> Perform a `GET` request with two parameters, `page` and `per_page` in the query
 	string.
-<2> Configure Spring REST Docs to  produce a snippet describing the request's parameters.
+<2> Configure Spring REST Docs to produce a snippet describing the request's parameters.
 	Uses the static `requestParameters` method on
 	`org.springframework.restdocs.request.RequestDocumentation`.
 <3> Document the `page` parameter. Uses the static `parameterWithName` method on
@@ -575,12 +575,58 @@ prevent it from appearing in the generated snippet while avoiding the failure de
 above.
 
 
-[[documenting-your-api-request-parts]]
-=== Request parts content
+[[documenting-your-api-request-parts-payloads]]
+=== Request part payloads
 
-If you need to document the content of request part that holds Json data, you should use`requestPartsFields`.
-It acts exactly as [[documenting-your-api-request-parts]] but in order to use it, you must specify the part name.
+The payload of a request part can be documented in much the same way as the
+<<documenting-your-api-request-response-payloads,payload of a request>>:
 
+[source,java,indent=0,role="primary"]
+.MockMvc
+----
+include::{examples-dir}/com/example/mockmvc/RequestPartPayload.java[tags=payload]
+----
+<1> Configure Spring REST docs to produce a snippet describing the fields in the payload
+	of the request part named `metadata`. Uses the static `requestPartFields` method on
+	`PayloadDocumentation`.
+	payload.
+<2> Expect a field with the path `version`. Uses the static `fieldWithPath` method on
+	`org.springframework.restdocs.payload.PayloadDocumentation`.
+
+[source,java,indent=0,role="secondary"]
+.REST Assured
+----
+include::{examples-dir}/com/example/restassured/RequestPartPayload.java[tags=payload]
+----
+<1> Configure Spring REST docs to produce a snippet describing the fields in the payload
+	of the request part named `metadata`. Uses the static `requestPartFields` method on
+	`PayloadDocumentation`.
+<2> Expect a field with the path `version`. Uses the static `fieldWithPath` method on
+	`org.springframework.restdocs.payload.PayloadDocumentation`.
+
+The result is a snippet that contains a table describing the part's fields. This snippet
+is named `request-part-${part-name}-fields.adoc`. For example, documenting a part named
+`metadata` will produce a snippet named `request-part-metadata-fields.adoc`.
+
+When documenting fields, the test will fail if an undocumented field is found in the
+payload of the part. Similarly, the test will also fail if a documented field is not found
+in the payload of the part and the field has not been marked as optional. For payloads
+with a hierarchical structure, documenting a field is sufficient for all of its
+descendants to also be treated as having been documented.
+
+If you do not want to document a field, you can mark it as ignored. This will prevent it
+from appearing in the generated snippet while avoiding the failure described above.
+
+Fields can also be documented in a relaxed mode where any undocumented fields will not
+cause a test failure. To do so, use the `relaxedRequestPartFields` method on
+`org.springframework.restdocs.payload.PayloadDocumentation`. This can be useful when
+documenting a particular scenario where you only want to focus on a subset of the payload
+of the part.
+
+For further information on describing fields, documenting payloads that use XML,
+and more please refer to the
+<<documenting-your-api-request-response-payloads,section on documenting request and
+response payloads>>.
 
 [[documenting-your-api-http-headers]]
 === HTTP headers

docs/src/test/java/com/example/mockmvc/RequestPartPayload.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2014-2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.mockmvc;
+
+import org.springframework.http.MediaType;
+import org.springframework.mock.web.MockMultipartFile;
+import org.springframework.test.web.servlet.MockMvc;
+
+import static org.springframework.restdocs.mockmvc.MockMvcRestDocumentation.document;
+import static org.springframework.restdocs.mockmvc.RestDocumentationRequestBuilders.fileUpload;
+import static org.springframework.restdocs.payload.PayloadDocumentation.fieldWithPath;
+import static org.springframework.restdocs.payload.PayloadDocumentation.requestPartFields;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+public class RequestPartPayload {
+
+	private MockMvc mockMvc;
+
+	public void response() throws Exception {
+		// tag::payload[]
+		MockMultipartFile image = new MockMultipartFile("image", "image.png", "image/png",
+				"<<png data>>".getBytes());
+		MockMultipartFile metadata = new MockMultipartFile("metadata", "",
+				"application/json", "{ \"version\": \"1.0\"}".getBytes());
+
+		this.mockMvc.perform(fileUpload("/images").file(image).file(metadata)
+					.accept(MediaType.APPLICATION_JSON))
+			.andExpect(status().isOk())
+			.andDo(document("image-upload", requestPartFields("metadata", // <1>
+					fieldWithPath("version").description("The version of the image")))); // <2>
+		// end::payload[]
+	}
+
+}

docs/src/test/java/com/example/restassured/RequestPartPayload.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2014-2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.restassured;
+
+import java.io.File;
+import java.util.HashMap;
+import java.util.Map;
+
+import com.jayway.restassured.RestAssured;
+import com.jayway.restassured.specification.RequestSpecification;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.springframework.restdocs.payload.PayloadDocumentation.fieldWithPath;
+import static org.springframework.restdocs.payload.PayloadDocumentation.requestPartFields;
+import static org.springframework.restdocs.restassured.RestAssuredRestDocumentation.document;
+
+public class RequestPartPayload {
+
+	private RequestSpecification spec;
+
+	public void response() throws Exception {
+		// tag::payload[]
+		Map<String, String> metadata = new HashMap<>();
+		metadata.put("version", "1.0");
+		RestAssured.given(this.spec).accept("application/json")
+			.filter(document("image-upload", requestPartFields("metadata", // <1>
+					fieldWithPath("version").description("The version of the image")))) // <2>
+			.when().multiPart("image", new File("image.png"), "image/png")
+					.multiPart("metadata", metadata).post("images")
+			.then().assertThat().statusCode(is(200));
+		// end::payload[]
+	}
+
+}