[ML] Fix POST version of get categories API (elastic/x-pack-elasticsearch#1184)

Also improves PageParams parsing to fill in defaults.

relates elastic/x-pack-elasticsearch#1180

Original commit: elastic/x-pack-elasticsearch@fccd7795ca

plugin/src/main/java/org/elasticsearch/xpack/ml/action/util/PageParams.java

@@ -26,14 +26,14 @@ public class PageParams extends ToXContentToBytes implements Writeable {
     public static final int DEFAULT_SIZE = 100;
 
 
-    public static final ConstructingObjectParser<PageParams, Void> PARSER = new ConstructingObjectParser<>(
-            PAGE.getPreferredName(), a -> new PageParams((int) a[0], (int) a[1]));
+    public static final ConstructingObjectParser<PageParams, Void> PARSER = new ConstructingObjectParser<>(PAGE.getPreferredName(),
+            a -> new PageParams(a[0] == null ? DEFAULT_FROM : (int) a[0], a[1] == null ? DEFAULT_SIZE : (int) a[1]));
 
     public static final int MAX_FROM_SIZE_SUM = 10000;
 
     static {
-        PARSER.declareInt(ConstructingObjectParser.constructorArg(), FROM);
-        PARSER.declareInt(ConstructingObjectParser.constructorArg(), SIZE);
+        PARSER.declareInt(ConstructingObjectParser.optionalConstructorArg(), FROM);
+        PARSER.declareInt(ConstructingObjectParser.optionalConstructorArg(), SIZE);
     }
 
     private final int from;

plugin/src/main/java/org/elasticsearch/xpack/ml/rest/results/RestGetCategoriesAction.java

@@ -49,9 +49,10 @@ public class RestGetCategoriesAction extends BaseRestHandler {
         if (bodyBytes != null && bodyBytes.length() > 0) {
             XContentParser parser = restRequest.contentParser();
             request = GetCategoriesAction.Request.parseRequest(jobId, parser);
+            if (!Strings.isNullOrEmpty(categoryId)) {
                 request.setCategoryId(categoryId);
+            }
         } else {
-
             request = new Request(jobId);
             if (!Strings.isNullOrEmpty(categoryId)) {
                 request.setCategoryId(categoryId);

plugin/src/test/resources/rest-api-spec/test/ml/jobs_get_result_categories.yaml

@@ -49,7 +49,54 @@ setup:
   - match: { categories.1.category_id: 2 }
 
 ---
-"Test result category api":
+"Test get categories with pagination":
+  - do:
+      xpack.ml.get_categories:
+        job_id: "farequote"
+        size: 1
+
+  - length: { categories: 1 }
+  - match: { categories.0.job_id: farequote }
+  - match: { categories.0.category_id: 1 }
+
+  - do:
+     xpack.ml.get_categories:
+        job_id: "farequote"
+        from: 1
+        size: 2
+
+  - length: { categories: 1 }
+  - match: { categories.0.job_id: farequote }
+  - match: { categories.0.category_id: 2 }
+
+---
+"Test post get categories with pagination":
+  - do:
+      xpack.ml.get_categories:
+        job_id: "farequote"
+        body: >
+          {
+            "page": { "size": 1}
+          }
+
+  - length: { categories: 1 }
+  - match: { categories.0.job_id: farequote }
+  - match: { categories.0.category_id: 1 }
+
+  - do:
+     xpack.ml.get_categories:
+        job_id: "farequote"
+        body: >
+          {
+            "page": { "from":1, "size": 1}
+          }
+
+  - length: { categories: 1 }
+  - match: { categories.0.job_id: farequote }
+  - match: { categories.0.category_id: 2 }
+
+---
+"Test get category by id":
   - do:
       xpack.ml.get_categories:
         job_id: "farequote"