better failure message when failing to load _uid field

src/main/java/org/elasticsearch/index/mapper/internal/SourceFieldSelector.java

@@ -24,8 +24,6 @@ import org.elasticsearch.common.lucene.document.ResetFieldSelector;
 
 /**
  * An optimized field selector that loads just the uid.
- *
- *
  */
 public class SourceFieldSelector implements ResetFieldSelector {
 
@@ -46,4 +44,9 @@ public class SourceFieldSelector implements ResetFieldSelector {
     @Override
     public void reset() {
     }
+
+    @Override
+    public String toString() {
+        return "source";
+    }
 }

src/main/java/org/elasticsearch/index/mapper/selector/AllButSourceFieldSelector.java

@@ -25,8 +25,6 @@ import org.elasticsearch.index.mapper.internal.SourceFieldMapper;
 
 /**
  * A field selector that loads all fields except the source field.
- *
- *
  */
 public class AllButSourceFieldSelector implements ResetFieldSelector {
 
@@ -43,4 +41,9 @@ public class AllButSourceFieldSelector implements ResetFieldSelector {
     @Override
     public void reset() {
     }
+
+    @Override
+    public String toString() {
+        return "all_but_source";
+    }
 }

src/main/java/org/elasticsearch/index/mapper/selector/FieldMappersFieldSelector.java

@@ -54,4 +54,9 @@ public class FieldMappersFieldSelector implements ResetFieldSelector {
     @Override
     public void reset() {
     }
+
+    @Override
+    public String toString() {
+        return "fields(" + names + ")";
+    }
 }

src/main/java/org/elasticsearch/index/mapper/selector/UidAndRoutingFieldSelector.java

@@ -52,4 +52,9 @@ public class UidAndRoutingFieldSelector implements ResetFieldSelector {
     public void reset() {
         match = 0;
     }
+
+    @Override
+    public String toString() {
+        return "uid_and_routing";
+    }
 }

src/main/java/org/elasticsearch/index/mapper/selector/UidAndSourceFieldSelector.java

@@ -26,8 +26,6 @@ import org.elasticsearch.index.mapper.internal.UidFieldMapper;
 
 /**
  * An optimized field selector that loads just the uid and the source.
- *
- *
  */
 public class UidAndSourceFieldSelector implements ResetFieldSelector {
 
@@ -54,4 +52,9 @@ public class UidAndSourceFieldSelector implements ResetFieldSelector {
     public void reset() {
         match = 0;
     }
+
+    @Override
+    public String toString() {
+        return "uid_and_source";
+    }
 }

src/main/java/org/elasticsearch/index/mapper/selector/UidFieldSelector.java

@@ -25,8 +25,6 @@ import org.elasticsearch.index.mapper.internal.UidFieldMapper;
 
 /**
  * An optimized field selector that loads just the uid.
- *
- *
  */
 public class UidFieldSelector implements ResetFieldSelector {
 
@@ -47,4 +45,9 @@ public class UidFieldSelector implements ResetFieldSelector {
     @Override
     public void reset() {
     }
+
+    @Override
+    public String toString() {
+        return "uid";
+    }
 }

src/main/java/org/elasticsearch/search/fetch/FetchPhase.java

@@ -156,7 +156,7 @@ public class FetchPhase implements SearchPhase {
         for (int index = 0; index < context.docIdsToLoadSize(); index++) {
             int docId = context.docIdsToLoad()[context.docIdsToLoadFrom() + index];
             Document doc = loadDocument(context, fieldSelector, docId);
-            Uid uid = extractUid(context, doc);
+            Uid uid = extractUid(context, doc, fieldSelector);
 
             DocumentMapper documentMapper = context.mapperService().documentMapper(uid.type());
 
@@ -268,8 +268,7 @@ public class FetchPhase implements SearchPhase {
         return null;
     }
 
-    private Uid extractUid(SearchContext context, Document doc) {
-        // TODO we might want to use FieldData here to speed things up, so we don't have to load it at all...
+    private Uid extractUid(SearchContext context, Document doc, @Nullable ResetFieldSelector fieldSelector) {
         String sUid = doc.get(UidFieldMapper.NAME);
         if (sUid != null) {
             return Uid.createUid(sUid);
@@ -279,7 +278,7 @@ public class FetchPhase implements SearchPhase {
         for (Fieldable field : doc.getFields()) {
             fieldNames.add(field.name());
         }
-        throw new FetchPhaseExecutionException(context, "Failed to load uid from the index, missing internal _uid field, current fields in the doc [" + fieldNames + "]");
+        throw new FetchPhaseExecutionException(context, "Failed to load uid from the index, missing internal _uid field, current fields in the doc [" + fieldNames + "], selector [" + fieldSelector + "]");
     }
 
     private Document loadDocument(SearchContext context, @Nullable ResetFieldSelector fieldSelector, int docId) {