Enable rendering of empty list conditions

This will enable the list conditions to render invalid SQL, so it should
be used with caution. But it will make transition from some legacy code
bases easier.

Resolves #228

Author: jeffgbutler

src/main/java/org/mybatis/dynamic/sql/AbstractListValueCondition.java

@@ -1,5 +1,5 @@
 /**
- *    Copyright 2016-2019 the original author or authors.
+ *    Copyright 2016-2020 the original author or authors.
  *
  *    Licensed under the Apache License, Version 2.0 (the "License");
  *    you may not use this file except in compliance with the License.
@@ -25,6 +25,7 @@
 public abstract class AbstractListValueCondition<T> implements VisitableCondition<T> {
     protected Collection<T> values;
     protected UnaryOperator<Stream<T>> valueStreamTransformer;
+    protected boolean skipRenderingWhenEmpty = true;
 
     protected AbstractListValueCondition(Collection<T> values) {
         this(values, UnaryOperator.identity());
@@ -39,6 +40,10 @@ public final <R> Stream<R> mapValues(Function<T, R> mapper) {
         return valueStreamTransformer.apply(values.stream()).map(mapper);
     }
 
+    public boolean skipRenderingWhenEmpty() {
+        return skipRenderingWhenEmpty;
+    }
+    
     @Override
     public <R> R accept(ConditionVisitor<T, R> visitor) {
         return visitor.visit(this);

src/main/java/org/mybatis/dynamic/sql/where/render/WhereConditionVisitor.java

@@ -55,7 +55,7 @@ public Optional<FragmentAndParameters> visit(AbstractListValueCondition<T> condi
         FragmentCollector fc = condition.mapValues(this::toFragmentAndParameters)
                 .collect(FragmentCollector.collect());
 
-        if (fc.isEmpty()) {
+        if (fc.isEmpty() && condition.skipRenderingWhenEmpty()) {
             return Optional.empty();
         }
 

src/test/java/examples/animal/data/AnimalDataTest.java

@@ -17,6 +17,7 @@
 
 import static examples.animal.data.AnimalDataDynamicSqlSupport.*;
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.assertj.core.api.Assertions.within;
 import static org.junit.jupiter.api.Assertions.assertAll;
 import static org.mybatis.dynamic.sql.SqlBuilder.*;
@@ -26,10 +27,13 @@
 import java.sql.Connection;
 import java.sql.DriverManager;
 import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 
 import org.apache.ibatis.datasource.unpooled.UnpooledDataSource;
+import org.apache.ibatis.exceptions.PersistenceException;
 import org.apache.ibatis.jdbc.ScriptRunner;
 import org.apache.ibatis.mapping.Environment;
 import org.apache.ibatis.session.Configuration;
@@ -52,6 +56,7 @@
 import org.mybatis.dynamic.sql.select.render.SelectStatementProvider;
 import org.mybatis.dynamic.sql.update.render.UpdateStatementProvider;
 import org.mybatis.dynamic.sql.util.mybatis3.MyBatis3Utils;
+import org.mybatis.dynamic.sql.where.condition.IsIn;
 import org.mybatis.dynamic.sql.where.render.WhereClauseProvider;
 
 class AnimalDataTest {
@@ -564,6 +569,34 @@ void testInCondition() {
         }
     }
 
+    @Test
+    void testInConditionWithEmptyList() {
+        try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
+            AnimalDataMapper mapper = sqlSession.getMapper(AnimalDataMapper.class);
+            
+            SelectStatementProvider selectStatement = select(id, animalName, bodyWeight, brainWeight)
+                    .from(animalData)
+                    .where(id, IsInRequired.isIn(Collections.emptyList()))
+                    .build()
+                    .render(RenderingStrategies.MYBATIS3);
+
+            assertThatExceptionOfType(PersistenceException.class).isThrownBy(() -> {
+                mapper.selectMany(selectStatement);
+            });
+        }
+    }
+
+    public static class IsInRequired<T> extends IsIn<T> {
+        protected IsInRequired(Collection<T> values) {
+            super(values);
+            skipRenderingWhenEmpty = false;
+        }
+        
+        public static <T> IsInRequired<T> isIn(Collection<T> values) {
+            return new IsInRequired<>(values);
+        }
+    }
+    
     @Test
     void testInCaseSensitiveCondition() {
         try (SqlSession sqlSession = sqlSessionFactory.openSession()) {