JAVA-669: Setting $readPreference meta-operator for commands sent to mongos

Author: jyemin

src/main/com/mongodb/DB.java

@@ -64,9 +64,10 @@ public DB( Mongo mongo , String name ){
     }
 
     /**
-     * Tests if database commands are read preference obedient
-     * @param command the <code>DBObject</code> to test obedience
-     * @return true if the command is obedient
+     * Determines the read preference that should be used for the given command.
+     * @param command the <code>DBObject</code> representing the command
+     * @param requestedPreference the preference requested by the client.
+     * @return the read preference to use for the given command.  It will never return null.
      * @see com.mongodb.ReadPreference
      */
     ReadPreference getCommandReadPreference(DBObject command, ReadPreference requestedPreference){
@@ -93,6 +94,8 @@ ReadPreference getCommandReadPreference(DBObject command, ReadPreference request
 
         if (primaryRequired) {
             return ReadPreference.primary();
+        } else if (requestedPreference == null) {
+            return ReadPreference.primary();
         } else {
             return requestedPreference;
         }
@@ -253,9 +256,13 @@ public CommandResult command( DBObject cmd , int options, ReadPreference readPre
      * @dochub commands
      */
     public CommandResult command( DBObject cmd , int options, ReadPreference readPrefs, DBEncoder encoder ){
-
         readPrefs = getCommandReadPreference(cmd, readPrefs);
-        
+
+        // TODO: automate an integration test for this condition
+        if (getMongo().isMongosConnection()) {
+            cmd.put(QueryOpBuilder.READ_PREFERENCE_META_OPERATOR, readPrefs.toDBObject());
+        }
+
         Iterator<DBObject> i =
                 getCollection("$cmd").__find(cmd, new BasicDBObject(), 0, -1, 0, options, readPrefs ,
                         DefaultDBDecoder.FACTORY.create(), encoder);

src/main/com/mongodb/QueryOpBuilder.java

@@ -1,15 +1,13 @@
 package com.mongodb;
 
-import com.mongodb.BasicDBObject;
-import com.mongodb.DBObject;
-
-
 /**
  * Utility for constructing Query operation command with query, orderby, hint, explain, snapshot.
  */
 class QueryOpBuilder {
-	
-	private DBObject query;
+
+    static final String READ_PREFERENCE_META_OPERATOR = "$readPreference";
+
+    private DBObject query;
 	private DBObject orderBy;
 	private DBObject hintObj;
 	private String hintStr;
@@ -131,7 +129,7 @@ public DBObject get(){
             if (snapshot)
                 queryop.put("$snapshot", true);
             if (readPref != null)
-                queryop.put("$readPreference", readPref);
+                queryop.put(READ_PREFERENCE_META_OPERATOR, readPref);
 
             return queryop;
 		}

src/test/com/mongodb/DBTest.java

@@ -118,6 +118,9 @@ public void testReadPreferenceObedience() {
 
         obj = new BasicDBObject("serverStatus", 1);
         assertEquals(ReadPreference.primary(), _db.getCommandReadPreference(obj, ReadPreference.secondary()));
+
+        obj = new BasicDBObject("count", 1);
+        assertEquals(ReadPreference.primary(), _db.getCommandReadPreference(obj, null));
     }
 
     @Test(groups = {"basic"})