fix several bugs in findSingleAttibute (#980)

* add test case for distinct on id property

* suggested fix for "select distinct id"

* add test case for distinct with fetch

* FIX: .setDistinct(true) can be used in conjunction with fetch() now

* add test case for distinct with beans that have a disriminator column

* FIX: discriminator column is only read if also Id is read.

* fine tuned the test case

* added test cases for findSingleAttributeList without distinct

* improved fix to support also findSingleAttribute without distinct

* ADD: Bonus test case - assertion not yet verified

src/main/java/io/ebeaninternal/server/query/SqlTreeBuilder.java

@@ -322,7 +322,9 @@ public class SqlTreeBuilder {
       return new SqlTreeNodeManyRoot(prefix, (BeanPropertyAssocMany<?>) prop, props, myList, temporalMode, disableLazyLoad);
 
     } else {
-      return new SqlTreeNodeBean(prefix, prop, props, myList, temporalMode, disableLazyLoad);
+      // do not read Id on child beans (e.g. when used with fetch())
+      boolean withId = (query == null || !query.isSingleAttribute()); 
+      return new SqlTreeNodeBean(prefix, prop, props, myList, withId, temporalMode, disableLazyLoad);
     }
   }
 
@@ -438,7 +440,7 @@ public class SqlTreeBuilder {
         p = desc.findBeanProperty("id");
         selectProps.add(p);
 
-      } else if (p.isId()) {
+      } else if (p.isId() && (query == null || !query.isSingleAttribute())) {
         // do not bother to include id for normal queries as the
         // id is always added (except for subQueries)
 

src/main/java/io/ebeaninternal/server/query/SqlTreeNodeBean.java

@@ -58,7 +58,7 @@ class SqlTreeNodeBean implements SqlTreeNode {
    * False if report bean and has no id property.
    */
   protected final boolean readId;
-
+  
   private final boolean disableLazyLoad;
 
   protected final InheritInfo inheritInfo;
@@ -89,9 +89,9 @@ class SqlTreeNodeBean implements SqlTreeNode {
    * Construct for leaf node.
    */
   SqlTreeNodeBean(String prefix, BeanPropertyAssoc<?> beanProp, SqlTreeProperties props,
-                  List<SqlTreeNode> myChildren, SpiQuery.TemporalMode temporalMode, boolean disableLazyLoad) {
+                  List<SqlTreeNode> myChildren, boolean withId, SpiQuery.TemporalMode temporalMode, boolean disableLazyLoad) {
 
-    this(prefix, beanProp, beanProp.getTargetDescriptor(), props, myChildren, true, null, temporalMode, disableLazyLoad);
+    this(prefix, beanProp, beanProp.getTargetDescriptor(), props, myChildren, withId, null, temporalMode, disableLazyLoad);
   }
 
   /**
@@ -135,6 +135,11 @@ class SqlTreeNodeBean implements SqlTreeNode {
 
   @Override
   public BeanProperty getSingleProperty() {
+    if (properties == null || properties.length == 0) {
+      // if we have no property ask first children (in a distinct select with join)
+      // if we have also no children, NPE happens anyway.
+      return children[0].getSingleProperty();
+    }
     return properties[0];
   }
 
@@ -162,9 +167,9 @@ class SqlTreeNodeBean implements SqlTreeNode {
   @Override
   public void buildRawSqlSelectChain(List<String> selectChain) {
     if (readId) {
-      if (desc.hasInheritance()) {
+      if (inheritInfo != null) {
         // discriminator column always proceeds id column
-        selectChain.add(getPath(prefix, desc.getInheritInfo().getDiscriminatorColumn()));
+        selectChain.add(getPath(prefix, inheritInfo.getDiscriminatorColumn()));
       }
       idBinder.buildRawSqlSelectChain(prefix, selectChain);
     }
@@ -434,11 +439,11 @@ class SqlTreeNodeBean implements SqlTreeNode {
       lazyLoadParent.addSelectExported(ctx, prefix);
     }
 
-    if (!subQuery && inheritInfo != null) {
-      ctx.appendColumn(inheritInfo.getDiscriminatorColumn());
-    }
-
     if (readId) {
+      if (!subQuery && inheritInfo != null) {
+        ctx.appendColumn(inheritInfo.getDiscriminatorColumn());
+      }
+
       appendSelectId(ctx, idBinder.getBeanProperty());
     }
     appendSelect(ctx, subQuery, properties);

src/main/java/io/ebeaninternal/server/query/SqlTreeNodeManyRoot.java

@@ -15,7 +15,7 @@ final class SqlTreeNodeManyRoot extends SqlTreeNodeBean {
 
   SqlTreeNodeManyRoot(String prefix, BeanPropertyAssocMany<?> prop, SqlTreeProperties props, List<SqlTreeNode> myList,
                       SpiQuery.TemporalMode temporalMode, boolean disableLazyLoad) {
-    super(prefix, prop, props, myList, temporalMode, disableLazyLoad);
+    super(prefix, prop, props, myList, true, temporalMode, disableLazyLoad);
     this.manyProp = prop;
   }
 

src/main/java/io/ebeaninternal/server/querydefn/DefaultOrmQuery.java

@@ -48,6 +48,7 @@ import java.util.ArrayList;
 import java.util.Collection;
 import java.util.HashMap;
 import java.util.Iterator;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
@@ -1220,6 +1221,10 @@ public class DefaultOrmQuery<T> implements SpiQuery<T> {
   @Override
   @SuppressWarnings("unchecked")
   public <A> List<A> findSingleAttributeList() {
+    if (!detail.hasSelectClause()) {
+      // (no explicit select set - clear all properties)
+      detail.setBase(new OrmQueryProperties(null, new LinkedHashSet<>()));
+    }
     return (List<A>) server.findSingleAttributeList(this, null);
   }
 

src/main/java/io/ebeaninternal/server/querydefn/OrmQueryDetail.java

@@ -440,7 +440,7 @@ public class OrmQueryDetail implements Serializable {
     }
   }
 
-  private boolean hasSelectClause() {
+  public boolean hasSelectClause() {
     return baseProps.hasSelectClause();
   }