在MyBatis中以不同关联两次映射同一张表

Question

I have 6 tables structured like this (there is a more descriptive example at the bottom):

A

Every item is related to one item in B and one item in C.

aid | bid | cid

B

Every item is related to multiple items in D.

bid

C

Every item is related to multiple items in D.

cid

D

Every item is related to multiple items in B and C.

did

E

Maps items in B to items in D.

bid | did

F

Maps items in C to items in D.

cid | did

Now I'm trying to write a mapper in MyBatis that maps the items in table A. The mapper has an association to the mappers of table B and C, and the mappers of tables B and C both have a collection containing items of table D.

Individually mapping either B or C to items of table D isn't a problem, since then a simple join operation using either E or F is enough. However, the problem arises when trying to map everything from the perspective of table A. As table A is associated to both table B and C, and they are both associated with table D, I don't know how to let the mappers of B and C distinguish between the items in table D that are meant for them.

Basically, I'm trying to join D to B and C from the perspective of A's mapper.

Is there a way to join table D twice in MyBatis, whilst keeping a distinction between the two joins so the mappers of B and C can distinguish between the two?

Thanks for the help!

EDIT

Here's a small example with more descriptive names.

houses

houseid | livingroomid | kitchenid
1       | 1            | 1

livingrooms

livingroomid
1

kitchens

kitchenid
1

furniture

furnitureid | furniturename
1           | couch
2           | lamp
3           | fridge

livingroomfurniture

livingroomid | furnitureid
1            | 1
1            | 2

kitchenfurniture

kitchenid | furnitureid
1         | 2
1         | 3

My mappers currently:

Mapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper>
    <resultMap type="com.project.models.house.House" id="HouseResult">
        <id             property="id"           column="houseid" />
        <association    property="livingroom"   resultMap="LivingRoomResult" />
        <association    property="kitchen"      resultMap="KitchenResult" />
    </resultMap>

    <resultMap type="com.project.models.livingroom.LivingRoom" id="LivingRoomResult">
        <id             property="id"           column="livingroomid" />
        <collection     property="furniture"    resultMap="FurnitureResult" />
    </resultMap>

    <resultMap type="com.project.models.kitchen.Kitchen" id="KitchenResult">
        <id             property="id"           column="kitchenid" />
        <collection     property="furniture"    resultMap="FurnitureResult" />
    </resultMap>

    <resultMap type="com.project.models.furniture.Furniture" id="FurnitureResult">
        <id             property="id"           column="furnitureid" />
        <result         property="name"         column="furniturename" />
    </resultMap>

    <select id="getAllHouses" resultMap="HouseResult">
        SELECT
          houses.houseid,
          houses.livingroomid,
          houses.kitchenid

        FROM houses

        -- Get the living room
        INNER JOIN livingrooms ON livingrooms.livingroomid = houses.livingroomid

        -- Get the kitchen
        INNER JOIN kitchens ON kitchens.kitchenid = kitchens.kitchenid

        -- Here somehow join the furniture for both the living room and the kitchen?
    </select>
</mapper>

Answer 1

You actually have to join D to both tables B and C alias them in accordingly.

Let's say bd for table d joined to B and cd .

now if you want same resultmap of table d for both aliased table (bd and cd), you can do this:

select bd.did as bd_did, cd.did as cd_did

And if you have not come across column_prefix property of result maps this is how your resultmap will look

<collection type="D" resultMap="d" column_prefix="bd_" />