parameters in sql fragment substitute error - mybatis

Question

I write a sql fragment, like this:
<sql id="day">
<choose>
<when test="${property} == 'day'">
substring(datachange_createtime, 1, 10)
</when>
<otherwise>
${property}
</otherwise>
</choose>
</sql>
and include this sql like this:
select * from my_table
<if test="groupBy != null">
group by
<foreach collection="groupBy" item="groupByAttr" separator=",">
<include refid="day">
<property name="property" value="groupByAttr"/>
</include>
</foreach>
</if>
But I got an error, mybatis didn't substitute property with a real parameter, but with groupByAttr. The result sql is like:
select * from my_table group by groupByAttr
In the otherwise clause, if I replace ${property} with #{${property}}, The result sql becomes:
select * from my_table group by ?
It is indeed correct, but for some reasons, I don't want a prepared sql.
I think the problem is the behavior of substitution in test and otherwise clauses is different.
Is the a bug, or just some mistakes I took? How to solve this problem?

Answer 1

Short answer:
You need to change the <include /> as follows.
<sql id="day">
<choose>
<when test="${property} == 'day'">
substring(datachange_createtime, 1, 10)
</when>
<otherwise>
\\${${property}}
</otherwise>
</choose>
</sql>
Long answer:
With your original code, the statement will look as follows after the <include /> substitution.
select * from my_table
<if test="groupBy != null">
group by
<foreach collection="groupBy" item="groupByAttr" separator=",">
<choose>
<when test="groupByAttr == 'day'">
substring(datachange_createtime, 1, 10)
</when>
<otherwise>
groupByAttr
</otherwise>
</choose>
</foreach>
</if>
As you can see, <when /> is OK, but <otherwise /> is not.
<otherwise /> should look like this:
<otherwise>
${groupByAttr}
</otherwise>
In the expression in the short answer \\${${property}}, the first two backslashes is the escape syntax which avoids the outer ${} from being treated as a variable.
Note that #{groupByAttr} does not work here because, in java.sql.PreparedStatement, a placeholder ? is not allowed for a table/column name.
Please see the wiki page for the details.