Added new selectMap method to get map with specified property as key and value

[NiazBinSiraj]

Description:
A new selectMap() method has been added to SqlSession.java. This enhancement allows the conversion of a list of results into a Map, where the key is one specified property, and the value is another specified property from the resulting objects.

Background Story:
While working on a recent project in our company, I encountered a scenario where I needed to retrieve database results as a Map, with the key and value being specific column values. However, the current implementation of MyBatis does not support such a method. To address this, I’ve added the new selectMap() method, which I believe would be a valuable addition to the framework.

[harawata]

Thank you for the PR, @NiazBinSiraj !

Many people want this feature actually (e.g. #2672 #2865), however, it can easily be done using stream.

Map<Integer, String> map = mapper.selectAllAuthors().stream()
  .collect(Collectors.toMap(x-> x.getId(), x -> x.getUsername()));

You don't even need to define a custom class for this.

@Select("select * from author")
List<Map<String, Object>> selectMaps();

Map<Integer, String> map = mapper.selectMaps().stream()
  .collect(Collectors.toMap(e -> (Integer) e.get("id"), e -> (String) e.get("username")));

Also, for convenience, you can add a default method to the mapper interface.

default Map<Integer, String> getIdVsUsernameMap() {
  return selectMaps().stream()
    .collect(Collectors.toMap(e -> (Integer) e.get("id"), e -> (String) e.get("username")));
}

Although it requires one extra step, I think it's a better solution because it can handle more complex usages (and it does not require new methods in SqlSession).

I am going to close this as a duplicate.
Consider upvoting #2863 or #2865 if you still want to save the extra step.