repo-sqale: javadoc for ExtensionItemSqlMapper and ExtensionMapping

Also stateless ExtensionItemSqlMapper is instantiated in constructor
of ExtensionMapping, not every time in getItemMapper().

repo/repo-sqale/src/main/java/com/evolveum/midpoint/repo/sqale/mapping/ExtensionItemSqlMapper.java

@@ -24,7 +24,11 @@
 import com.evolveum.midpoint.repo.sqlbase.querydsl.FlexibleRelationalPathBase;
 
 /**
- * TODO: this much "lazier" than normal item mapper, we'll get to ext definitions only here, not sooner.
+ * Extension item mapper that is much lazier than {@link SqaleItemSqlMapper} for typical column.
+ * Normally the mapper knows how to get from the query/update contextual information to the columns.
+ * This mapper only knows the way to the JSONB column and lets extension item filter/delta
+ * processors to do the rest of the work based on the context and item information contained in
+ * the filter/modification.
  *
  * @param <Q> entity path owning the mapped item
  * @param <R> row type with the mapped item
@@ -48,6 +52,7 @@ public ExtensionItemSqlMapper(
         //  - sorting by ext does not make sense, we want to sort by ext->something (or ->>)
         //  - that also means that we don't want to return Path but Expression instead
         //  - but sorting by ext/something is not yet supported in SqlQueryContext.processOrdering
+        //  In short, we will not even get here
         return rootToExtensionPath.apply(entityPath);
     }
 

repo/repo-sqale/src/main/java/com/evolveum/midpoint/repo/sqale/mapping/ExtensionMapping.java

@@ -22,8 +22,8 @@
 /**
  * This acts like a container mapping for extension/attributes containers.
  * Compared to other subclasses of {@link QueryModelMapping} this does NOT use the item mapping
- * and resolver maps, but instead creates the mapper on the fly with currently available information
- * and lets the extension mapper/resolver do the real work.
+ * and resolver maps, but instead returns the same stateless mapper which then delegates
+ * all the real work to item filter/delta processors (see {@link ExtensionItemSqlMapper}).
  * This allows for dynamic mapping which is needed especially for shadow attributes.
  *
  * @param <Q> type of entity path
@@ -33,7 +33,7 @@ public class ExtensionMapping<Q extends FlexibleRelationalPathBase<R>, R>
         extends QueryModelMapping<Containerable, Q, R> {
 
     private final MExtItemHolderType holderType;
-    private final Function<Q, JsonbPath> rootToExtensionPath;
+    private final ExtensionItemSqlMapper<Q, R> itemMapper;
 
     protected ExtensionMapping(
             @NotNull MExtItemHolderType holderType,
@@ -42,12 +42,12 @@ protected ExtensionMapping(
         super(Containerable.class, queryType);
 
         this.holderType = holderType;
-        this.rootToExtensionPath = rootToExtensionPath;
+        this.itemMapper = new ExtensionItemSqlMapper<>(rootToExtensionPath, holderType);
     }
 
     @Override
     public @Nullable ItemSqlMapper<Q, R> getItemMapper(QName itemName) {
-        return new ExtensionItemSqlMapper<>(rootToExtensionPath, holderType);
+        return itemMapper;
     }
 
     public MExtItemHolderType holderType() {