Added ability to set alias for root table of sql query.

src/main/java/com/avaje/ebean/Query.java

@@ -1230,4 +1230,9 @@ public interface Query<T> extends Serializable {
    * Return true if this query has forUpdate set.
    */
   public boolean isForUpdate();
+  
+  /**
+   * Set root table alias.
+   */
+  public Query<T> alias(String alias);
 }

src/main/java/com/avaje/ebeaninternal/api/SpiQuery.java

@@ -611,4 +611,9 @@ public interface SpiQuery<T> extends Query<T> {
      * Return true if this query has been cancelled.
      */
     public boolean isCancelled();
+    
+    /**
+     * Return root table alias set by {@link #alias(String)} command.
+     */
+    public String getAlias();
 }

src/main/java/com/avaje/ebeaninternal/server/query/SqlTreeBuilder.java

@@ -104,7 +104,7 @@ public class SqlTreeBuilder {
     this.queryDetail = query.getDetail();
 
     this.predicates = predicates;
-    this.alias = new SqlTreeAlias(request.getBeanDescriptor().getBaseTableAlias());
+    this.alias = new SqlTreeAlias(request.getQuery().getAlias()==null?request.getBeanDescriptor().getBaseTableAlias():request.getQuery().getAlias());
     this.ctx = new DefaultDbSqlContext(alias, tableAliasPlaceHolder, columnAliasPrefix, !subQuery);
   }
 

src/main/java/com/avaje/ebeaninternal/server/querydefn/DefaultOrmQuery.java

@@ -190,6 +190,11 @@ public class DefaultOrmQuery<T> implements SpiQuery<T> {
   private boolean autoFetchTuned;
 
   private boolean logSecondaryQuery;
+  
+  /**
+   * Root table alias. For {@link Query#alias(String)} command.
+   */
+  private String rootTableAlias=null;
 
   /**
    * The node of the bean or collection that fired lazy loading. Not null if profiling is on and
@@ -680,6 +685,7 @@ public class DefaultOrmQuery<T> implements SpiQuery<T> {
     builder.add(id != null);
     builder.add(rawSql == null ? 0 : rawSql.queryHash());
     builder.add(includeTableJoin != null ? includeTableJoin.queryHash() : 0);
+    builder.add(rootTableAlias);
 
     if (detail != null) {
       detail.queryPlanHash(request, builder);
@@ -1284,6 +1290,17 @@ public class DefaultOrmQuery<T> implements SpiQuery<T> {
       this.cancelableQuery = cancelableQuery;
     }
   }
+  
+  @Override
+  public Query<T> alias(String alias) {
+	  this.rootTableAlias=alias;
+	  return this;
+  }
+  
+  @Override
+  public String getAlias() {
+    return rootTableAlias;
+  }
 
   public void cancel() {
     synchronized (this) {

src/test/java/com/avaje/tests/query/TestQueryAlias.java

@@ -0,0 +1,35 @@
+package com.avaje.tests.query;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import com.avaje.ebean.BaseTestCase;
+import com.avaje.ebean.Ebean;
+import com.avaje.ebean.Query;
+import com.avaje.tests.model.basic.CKeyParent;
+import com.avaje.tests.model.basic.ResetBasicData;
+
+public class TestQueryAlias extends BaseTestCase {
+	@Test
+	public void testExists() {
+		ResetBasicData.reset();
+		
+	    Query<CKeyParent> sq = Ebean.createQuery(CKeyParent.class).select("id.oneKey").alias("st0")
+	            .setAutofetch(false).where().query();
+
+	        Query<CKeyParent> pq = Ebean.find(CKeyParent.class).alias("myt0").where().in("id.oneKey", sq).query();
+
+	        pq.findList();
+
+	        String sql = pq.getGeneratedSql();
+	        
+	        System.out.println(sql);
+	        // Without alias command is should be:
+	        // select t0.one_key c0, t0.two_key c1, t0.name c2, t0.version c3, t0.assoc_id c4 from ckey_parent t0 where  (t0.one_key) in (select t0.one_key from ckey_parent t0)
+	        // but with alias command SQL should look like this:
+	        // select myt0.one_key c0, myt0.two_key c1, myt0.name c2, myt0.version c3, myt0.assoc_id c4 from ckey_parent myt0 where  (myt0.one_key) in (select st0.one_key from ckey_parent st0) 
+	        
+	        Assert.assertTrue(sql.indexOf("ckey_parent myt0")>0);
+	        Assert.assertTrue(sql.indexOf("in (select st0.one_key from ckey_parent st0)")>0);
+	}
+}