I’ve been experiencing this ‘problem’ too. I currently use the left joins solution when there are <=3 tables, otherwise I just query the type and perform a single join, that seems to be faster than paying the overhead associated with the left joins.
Sent: vrijdag 23 maart 2007 22:22
Subject: RE: Polymorphism question
You can always do some left joins with the id. Something like.
Select id, col1, col2, type, colX, colY, colZ, colW
From BaseTable LEFT JOIN ChildTable1 ON BaseTable.id = ChildTable1.id LEFT JOIN ChildTable2 ON BaseTable.id = ChildTable2.id
Then any resultMap for these will work since the columns must be there if the map reads it, otherwise columns are unnecessary.
This approch seems to be the only thing that would work with a join.
Still, you can do a twist with discriminator tag.
<subMap value="1" resultMap="child1">
<subMap value="2" resultMap="child2">
<result property="subClass" property="id" select="getChildInfo1"/>
<result property="subClass" property="id" select="getChildInfo2"/>
By making sure the subClass calls setters of your main class, you can force a second select to be call to populate values from child table.
I doudt you can make a reference to the populating object itself (being able to write "this" would be a cool improvement!) inside the disciminator when you would like a second call to database...
<result property="this" property="id" select="getChildInfo1"/>
Sent: Friday, 23 March 2007 14:49
Subject: Polymorphism question
I have a polymorphism problem which I'm not sure is solvable by a pure iBATIS solution.
I have a fixed database model which represents a hierarchy(very simplified):
| id |col1 | col2 | type |
| id |colX | colY |
| id |colZ | colW |
Which has an obvious object hierarchy, and "type" contains a either 'A' or 'B' indicating which child table should be used.
The id of the base and the child is the same, and the same id can not exists in the two child tables.
I have found information which shows how the <discriminator> element can be used with a column indicating the object to create. Which solves half of my problem.
The other half is something which I don't think is solvable, but figured I'd ask just in case it is.
I need the select join statement to be modified depending on the value of the "type" column, which table to join with. I have no idea how to do this in iBATIS, and am currently leaning towards implementing this in Java logic (first get the type from the base table and then execute the correct select statement).
Any help appreciated,