Detailed explanation of MyBatis one-to-many query configuration: solving common related query problems

MyBatis one-to-many query configuration detailed explanation: to solve common related query problems, specific code examples are needed

In actual development work, we often encounter the need to query the main The case of an entity object and its associated multiple slave entity objects. In MyBatis, one-to-many query is a common database association query. With correct configuration, the query, display and operation of associated objects can be easily realized. This article will introduce the configuration method of one-to-many query in MyBatis, and how to solve some common related query problems. It will also provide specific code examples.

1. Definition of one-to-many query

In a database, a one-to-many relationship usually means that a piece of data in a master table corresponds to data in multiple slave tables. In object relational mapping (ORM), a one-to-many relationship can be expressed as a relationship between a master entity object and multiple slave entity objects. In MyBatis, one-to-many queries can be implemented by defining SQL mapping files.

2. Configuration method of one-to-many query

In MyBatis, one-to-many query can be passed through the <collection></collection> tag and <select> </select> tag to achieve. There are usually two ways to configure a one-to-many query:

(1) Use the <collection></collection> tag

by using ## in the resultMap of the main entity object # tag to configure one-to-many query, the example is as follows:

<resultMap id="blogResultMap" type="Blog">
    <id property="id" column="id"/>
    <result property="title" column="title"/>
    <result property="content" column="content"/>
    <collection property="comments" ofType="Comment">
        <id property="id" column="id"/>
        <result property="content" column="content"/>
    </collection>
</resultMap>

(2) Use the

tag through the main Use the

tag in the resultMap of the entity object to configure a one-to-many query. The example is as follows:

<resultMap id="blogResultMap" type="Blog">
    <id property="id" column="id"/>
    <result property="title" column="title"/>
    <result property="content" column="content"/>
    <select property="comments" resultMap="commentResultMap">
        SELECT * FROM comments WHERE blog_id = #{id}
    </select>
</resultMap>

3. Solve common related query problems

When performing a When performing multiple queries, you may encounter some common problems, such as lazy loading, N 1 queries, etc. The following are solutions to these problems:

(1) Lazy loading

MyBatis supports the lazy loading mechanism. You can enable lazy loading by setting the

lazyLoadingEnabled attribute. The example is as follows :

<settings>
    <setting name="lazyLoadingEnabled" value="true"/>
</settings>

(2) N 1 query

N 1 query refers to the situation that when querying the master entity object, it will cause additional N queries to the slave entity object. The N 1 query problem can be solved by using the

fetchType="lazy" attribute in the tag. The example is as follows:

<collection property="comments" ofType="Comment" fetchType="lazy">
    <id property="id" column="id"/>
    <result property="content" column="content"/>
</collection>

4. Code example

The following is a simple example that demonstrates how to use MyBatis to configure one-to-many queries:

public interface BlogMapper {
    
    Blog selectBlogWithComments(int id);
    
}

<select id="selectBlogWithComments" resultMap="blogResultMap">
    SELECT * FROM blogs WHERE id = #{id}
</select>

public class Blog {
    
    private int id;
    private String title;
    private String content;
    private List<Comment> comments;
    
    // 省略getter和setter方法
}

public class Comment {
    
    private int id;
    private String content;
    
    // 省略getter和setter方法
}

In the above example,

Blog and Comment Represent blogs and comments respectively. Blog objects with comments can be queried through the selectBlogWithComments method.

Conclusion

This article introduces the configuration method of one-to-many query in MyBatis, solves some common related query problems, and provides specific code examples. In actual development, reasonable configuration of one-to-many queries can effectively improve the efficiency and accuracy of data queries. I hope this article can be helpful to readers.

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