More Like This Query: defaults to all possible fields for items

Items with no specified field now defaults to all the possible fields from the
document source. Previously, we had required 'fields' to be specified either
as a top level parameter or for each item. The default behavior is now similar
to the MLT API.

Closes #7382

docs/reference/query-dsl/queries/mlt-query.asciidoc

@@ -46,7 +46,6 @@ If only one document is specified, the query behaves the same as the
 }
 --------------------------------------------------
 
-
 `more_like_this` can be shortened to `mlt`.
 
 Under the hood, `more_like_this` simply creates multiple `should` clauses in a `bool` query of
@@ -61,26 +60,29 @@ such as `min_word_length`, `max_word_length` or `stop_words`, to control what
 terms should be considered as interesting. In order to give more weight to
 more interesting terms, each boolean clause associated with a term could be
 boosted by the term tf-idf score times some boosting factor `boost_terms`.
-
 When a search for multiple `docs` is issued, More Like This generates a
 `more_like_this` query per document field in `fields`. These `fields` are
 specified as a top level parameter or within each `doc`.
 
+IMPORTANT: The fields must be indexed and of type `string`. Additionally, when
+using `ids` or `docs`, the fields must be either `stored`, store `term_vector`
+or `_source` must be enabled.
+
 The `more_like_this` top level parameters include:
 
 [cols="<,<",options="header",]
 |=======================================================================
 |Parameter |Description
 |`fields` |A list of the fields to run the more like this query against.
-Defaults to the `_all` field.
+Defaults to the `_all` field for `like_text` and to all possible fields
+for `ids` or `docs`.
 
 |`like_text` |The text to find documents like it, *required* if `ids` or `docs` are
 not specified.
 
 |`ids` or `docs` |A list of documents following the same syntax as the 
-<<docs-multi-get,Multi GET API>>. This parameter is *required* if
-`like_text` is not specified. The texts are fetched from `fields` unless
-specified in each `doc`, and cannot be set to `_all`.
+<<docs-multi-get,Multi GET API>>. The text is fetched from `fields`
+unless specified otherwise in each `doc`.
 
 |`include` |When using `ids` or `docs`, specifies whether the documents should be
 included from the search. Defaults to `false`.

src/main/java/org/elasticsearch/index/query/MoreLikeThisQueryParser.java

@@ -164,28 +164,34 @@ public Query parse(QueryParseContext parseContext) throws IOException, QueryPars
         if (mltQuery.getLikeText() == null && items.isEmpty()) {
             throw new QueryParsingException(parseContext.index(), "more_like_this requires at least 'like_text' or 'ids/docs' to be specified");
         }
+        if (moreLikeFields != null && moreLikeFields.isEmpty()) {
+            throw new QueryParsingException(parseContext.index(), "more_like_this requires 'fields' to be non-empty");
+        }
 
+        // set analyzer
         if (analyzer == null) {
             analyzer = parseContext.mapperService().searchAnalyzer();
         }
         mltQuery.setAnalyzer(analyzer);
 
-        if (moreLikeFields == null) {
+        // set like text fields
+        boolean useDefaultField = (moreLikeFields == null);
+        if (useDefaultField) {
             moreLikeFields = Lists.newArrayList(parseContext.defaultField());
-        } else if (moreLikeFields.isEmpty()) {
-            throw new QueryParsingException(parseContext.index(), "more_like_this requires 'fields' to be non-empty");
         }
-
+        // possibly remove unsupported fields
         removeUnsupportedFields(moreLikeFields, analyzer, failOnUnsupportedField);
         if (moreLikeFields.isEmpty()) {
             return null;
         }
         mltQuery.setMoreLikeFields(moreLikeFields.toArray(Strings.EMPTY_ARRAY));
 
+        // support for named query
         if (queryName != null) {
             parseContext.addNamedQuery(queryName, mltQuery);
         }
 
+        // handle items
         if (!items.isEmpty()) {
             // set default index, type and fields if not specified
             for (MultiGetRequest.Item item : items) {
@@ -201,7 +207,11 @@ public Query parse(QueryParseContext parseContext) throws IOException, QueryPars
                     }
                 }
                 if (item.fields() == null && item.fetchSourceContext() == null) {
-                    item.fields(moreLikeFields.toArray(new String[moreLikeFields.size()]));
+                    if (useDefaultField) {
+                        item.fields("*");
+                    } else {
+                        item.fields(moreLikeFields.toArray(new String[moreLikeFields.size()]));
+                    }
                 }
             }
             // fetching the items with multi-termvectors API