Add an example of converting a select statement to a count statement

src/test/java/examples/simple/ReusableWhereTest.java

@@ -17,9 +17,13 @@
 
 import static examples.simple.PersonDynamicSqlSupport.id;
 import static examples.simple.PersonDynamicSqlSupport.occupation;
+import static examples.simple.PersonDynamicSqlSupport.person;
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.entry;
 import static org.mybatis.dynamic.sql.SqlBuilder.isEqualTo;
+import static org.mybatis.dynamic.sql.SqlBuilder.isLessThan;
 import static org.mybatis.dynamic.sql.SqlBuilder.isNull;
+import static org.mybatis.dynamic.sql.SqlBuilder.select;
 import static org.mybatis.dynamic.sql.SqlBuilder.where;
 
 import java.io.InputStream;
@@ -38,6 +42,12 @@
 import org.apache.ibatis.transaction.jdbc.JdbcTransactionFactory;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.mybatis.dynamic.sql.render.RenderingStrategies;
+import org.mybatis.dynamic.sql.select.QueryExpressionModel;
+import org.mybatis.dynamic.sql.select.SelectModel;
+import org.mybatis.dynamic.sql.select.SubQuery;
+import org.mybatis.dynamic.sql.select.aggregate.CountAll;
+import org.mybatis.dynamic.sql.select.render.SelectStatementProvider;
 import org.mybatis.dynamic.sql.where.WhereApplier;
 
 class ReusableWhereTest {
@@ -114,5 +124,64 @@ void testUpdate() {
         }
     }
 
+    @Test
+    void testTransformToCount() {
+        try (SqlSession session = sqlSessionFactory.openSession()) {
+            PersonMapper mapper = session.getMapper(PersonMapper.class);
+
+            SelectModel selectModel = select(PersonMapper.selectList)
+                    .from(person)
+                    .where(id, isLessThan(5))
+                    .limit(2)
+                    .build();
+
+            SelectStatementProvider selectStatement = selectModel.render(RenderingStrategies.MYBATIS3);
+
+            assertThat(selectStatement.getSelectStatement()).isEqualTo(
+                    "select id as A_ID, first_name, last_name, birth_date, employed, occupation, address_id from Person where id < #{parameters.p1,jdbcType=INTEGER} limit #{parameters.p2}");
+            assertThat(selectStatement.getParameters()).containsOnly(entry("p1", 5), entry("p2", 2L));
+
+            SelectModel countModel = toCount(selectModel);
+            SelectStatementProvider countStatement = countModel.render(RenderingStrategies.MYBATIS3);
+
+            assertThat(countStatement.getSelectStatement()).isEqualTo(
+                    "select count(*) from (select id as A_ID, first_name, last_name, birth_date, employed, occupation, address_id from Person where id < #{parameters.p1,jdbcType=INTEGER})");
+            assertThat(countStatement.getParameters()).containsOnly(entry("p1", 5));
+
+            long count = mapper.count(countStatement);
+
+            assertThat(count).isEqualTo(4);
+        }
+
+    }
+
     private final WhereApplier commonWhere = where(id, isEqualTo(1)).or(occupation, isNull()).toWhereApplier();
+
+    /**
+     * This function transforms a select statement into a count statement by wrapping the select statement into
+     * a subquery. This can be used to create a single select statement and use it for both selects and counts
+     * in a paging scenario. This is more appropriate than a reusable where clause if the query is complex. For simple
+     * queries, a reusable where clause is best.
+     *
+     * <p>This function will strip any paging configuration, waits, order bys, etc. from the top level query. This
+     * will allow usage of a paging query for selects, and the transformed query for a count of all rows.
+     *
+     * @param selectModel the select model to transform
+     * @return a new select model that is "select count(*) from (subquery)" where subquery is the input select statement
+     */
+    static SelectModel toCount(SelectModel selectModel) {
+        // remove any paging configuration, order by, wait clause, etc. from the incoming select model
+        SelectModel strippedSelectModel = SelectModel.withQueryExpressions(selectModel.queryExpressions().toList())
+                .withStatementConfiguration(selectModel.statementConfiguration())
+                .build();
+
+        QueryExpressionModel model = QueryExpressionModel
+                .withSelectList(List.of(new CountAll()))
+                .withTable(new SubQuery.Builder().withSelectModel(strippedSelectModel).build())
+                .build();
+
+        return SelectModel.withQueryExpressions(List.of(model))
+                .withStatementConfiguration(selectModel.statementConfiguration())
+                .build();
+    }
 }