Mybatis implements one-to-one association mapping instance code

preface

In the objective world, objects rarely exist alone, such as the relationship between classes and students, the relationship between students and courses, and their instances can access each other, which is the correlation. Mybatis association mapping can greatly simplify the access of persistence layer data. The classification of association relations is as follows:

We first draw a simplified E-R diagram to represent the three relationships.

The above figure shows three relationships:

introduction

In actual project development, there is often a one-to-one relationship. For example, a person corresponds to an ID card information, which is a one-to-one relationship. Here is a simple example

example:

1. I will omit the process of creating a table, mainly a person table and an IDcard table. See step 2pojo class attributes for related attributes;

2. Create a person object and an IDcard object:

mybatis/pri/xiaoyang/otot/pojo/IDCard. java

mybatis/pri/xiaoyang/otot/pojo/Person. java

3. Next comes the XML Mapping File:

mybatis/pri/xiaoyang/otot/mapper/IDCardMapper. xml

mybatis/pri/xiaoyang/otot/mapper/PersonMapper. xml

Note: in person mapper XML defines a < select / > tag. The SQL statement of the tag queries the unique person information according to the primary key field of the person table. Because the person table is associated with the IDcard table, a mapping object with the resultmap value of personmap is returned. Personmap uses the < Association / > tag to map a one-to-one association relationship. The attribute select in the tag indicates that the selectidcardbyid defined in idcardmapper will be executed with the attribute value "card_id" of column as a parameter to query the corresponding IDcard data, and the queried data will be encapsulated in the "card" object of property attribute value.

summary

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