feat: 新增 一对多、多对一 查询功能

docs/.vitepress/config.ts

@@ -61,6 +61,7 @@ export default defineConfig({
                     {text: '增、删、改', link: '/zh/base/add-delete-update'},
                     {text: '查询和分页', link: '/zh/base/query'},
                     {text: '批量操作', link: '/zh/base/batch'},
+                    {text: '一对多、多对一', link: '/zh/base/field-query'},
                     {text: 'QueryWrapper', link: '/zh/base/querywrapper'},
                     {text: 'Db + Row', link: '/zh/base/db-row'},
                     {text: 'IService', link: '/zh/base/service'},

docs/zh/base/field-query.md

@@ -0,0 +1,88 @@
+# 一对多、多对一
+
+在很多场景下，我们可能会用到 `一对多`、`一对一`、`多对一`等场景的关联查询，MyBatis-Flex 内置了相关的方法，用于支持此类场景。
+
+## 代码示例
+
+以下是文章的示例，一篇文章可能归属于多个分类。
+
+```java
+public class Article {
+    private Long id;
+    private String title;
+    private String content;
+
+    //文章的归属分类，可能是 1 个或者多个
+    private List<Category> categories;
+
+    //getter setter
+}
+```
+
+查询代码如下：
+
+```java {10-13}
+QueryWrapper queryWrapper = QueryWrapper.create()
+        .select().form(ARTICLE)
+        .where(ARTICLE.id.ge(100));
+
+List<Article> articles = mapper.selectListByQuery(queryWrapper, fieldQueryBuilder -> {
+        fieldQueryBuilder
+        .field(Article::getCategories) // 或者 .field("categories")
+        .type(Category.class) //非集合，可以不指定 type
+        .queryWrapper(article -> QueryWrapper.create()
+            .select().from(CATEGORY)
+            .where(CATEGORY.id.in(
+                    select("category_id").from("article_category_mapping")
+                    .where("article_id = ?", article.getId())
+            )
+        ));
+    });
+```
+
+通过以上代码可以看出，`Article.categories` 字段的结果，来源于  `queryWrapper()` 方法构建的 `QueryWrapper`。
+
+其原理是：MyBatis-Flex 的内部逻辑是先查询出 `Article` 的数据，然后再根据 `Article` 构建出新的 SQL，查询分类，并赋值给 `Article.categories`，
+假设 `Article` 有 10 条数据，那么最终会进行 11 次数据库查询。
+
+查询的 SQL 大概如下：
+
+```sql
+select * from tb_article where id  >= 100;
+
+-- 以上 SQL 得到结果后，再执行查询分类的 SQL，如下：
+select * from tb_category where id in 
+(select category_id from article_category_mapping where article_id = 100);
+
+select * from tb_category where id in
+(select category_id from article_category_mapping where article_id = 101);
+
+select * from tb_category where id in
+(select category_id from article_category_mapping where article_id = 102);
+
+select * from tb_category where id in
+(select category_id from article_category_mapping where article_id = 103);
+
+select * from tb_category where id in
+(select category_id from article_category_mapping where article_id = 104);
+
+select * from tb_category where id in
+(select category_id from article_category_mapping where article_id = 105);
+
+select * from tb_category where id in
+(select category_id from article_category_mapping where article_id = 106);
+
+select * from tb_category where id in
+(select category_id from article_category_mapping where article_id = 107);
+
+select * from tb_category where id in
+(select category_id from article_category_mapping where article_id = 108);
+
+select * from tb_category where id in
+(select category_id from article_category_mapping where article_id = 109);
+```
+
+## 其他场景
+
+通过以上内容看出，`Article` 的任何属性，都是可以通过传入 `FieldQueryBuilder` 来构建 `QueryWrapper` 进行再次查询，
+这些不仅仅只适用于  `一对多`、`一对一`、`多对一`等场景。任何 `Article` 对象里的属性，需要二次查询赋值的，都是可以通过这种方式进行。

mybatis-flex-core/src/main/java/com/mybatisflex/core/field/FieldQuery.java

@@ -0,0 +1,51 @@
+/**
+ * Copyright (c) 2022-2023, Mybatis-Flex (fuhai999@gmail.com).
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.mybatisflex.core.field;
+
+import com.mybatisflex.core.query.QueryWrapper;
+
+import java.io.Serializable;
+
+public class FieldQuery implements Serializable {
+
+    private String field;
+    private Class<?> mappingType;
+    private QueryWrapper queryWrapper;
+
+    public String getField() {
+        return field;
+    }
+
+    public void setField(String field) {
+        this.field = field;
+    }
+
+    public Class<?> getMappingType() {
+        return mappingType;
+    }
+
+    public void setMappingType(Class<?> mappingType) {
+        this.mappingType = mappingType;
+    }
+
+    public QueryWrapper getQueryWrapper() {
+        return queryWrapper;
+    }
+
+    public void setQueryWrapper(QueryWrapper queryWrapper) {
+        this.queryWrapper = queryWrapper;
+    }
+}

mybatis-flex-core/src/main/java/com/mybatisflex/core/field/FieldQueryBuilder.java

@@ -0,0 +1,54 @@
+/**
+ * Copyright (c) 2022-2023, Mybatis-Flex (fuhai999@gmail.com).
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.mybatisflex.core.field;
+
+import com.mybatisflex.core.util.LambdaGetter;
+import com.mybatisflex.core.util.LambdaUtil;
+
+import java.io.Serializable;
+
+public class FieldQueryBuilder<T> implements Serializable {
+
+    private T entity;
+    private FieldQuery fieldQuery = new FieldQuery();
+
+    public FieldQueryBuilder(T entity) {
+        this.entity = entity;
+    }
+
+    public FieldQueryBuilder<T> field(String field){
+        fieldQuery.setField(field);
+        return this;
+    }
+
+    public FieldQueryBuilder<T> field(LambdaGetter<T> fn){
+        return field(LambdaUtil.getFieldName(fn));
+    }
+
+    public FieldQueryBuilder<T> type(Class<?> mappingType){
+        fieldQuery.setMappingType(mappingType);
+        return this;
+    }
+
+    public FieldQueryBuilder<T> queryWrapper(QueryBuilder<T> fun){
+        fieldQuery.setQueryWrapper(fun.build(entity));
+        return this;
+    }
+
+    public FieldQuery build() {
+        return fieldQuery;
+    }
+}

mybatis-flex-core/src/main/java/com/mybatisflex/core/field/QueryBuilder.java

@@ -0,0 +1,22 @@
+/**
+ * Copyright (c) 2022-2023, Mybatis-Flex (fuhai999@gmail.com).
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.mybatisflex.core.field;
+
+import com.mybatisflex.core.query.QueryWrapper;
+
+public interface QueryBuilder<T> {
+    QueryWrapper build(T entity);
+}