Ruby: Introduce API::getAnImmediateSubclass()

class A; end
    class B < A; end
    class C < B; end

In the example above, `getMember("A").getAnImmediateSubclass()` will
select only uses of B, whereas `getMember("A").getASubclass()` will
select uses of A, B and C. This is usually the behaviour you want.

ruby/ql/lib/codeql/ruby/ApiGraphs.qll

@@ -101,9 +101,26 @@ module API {
     DataFlow::ExprNode getAnInstantiation() { result = this.getInstance().getAnImmediateUse() }
 
     /**
-     * Gets a node representing a subclass of the class represented by this node.
+     * Gets a node representing a (direct or indirect) subclass of the class represented by this node.
+     * ```rb
+     * class A; end
+     * class B < A; end
+     * class C < B; end
+     * ```
+     * In the example above, `getMember("A").getASubclass()` will return uses of `A`, `B` and `C`.
      */
-    Node getASubclass() { result = this.getASuccessor(Label::subclass()) }
+    Node getASubclass() { result = this.getAnImmediateSubclass*() }
+
+    /**
+     * Gets a node representing a direct subclass of the class represented by this node.
+     * ```rb
+     * class A; end
+     * class B < A; end
+     * class C < B; end
+     * ```
+     * In the example above, `getMember("A").getAnImmediateSubclass()` will return uses of `B` only.
+     */
+    Node getAnImmediateSubclass() { result = this.getASuccessor(Label::subclass()) }
 
     /**
      * Gets a string representation of the lexicographically least among all shortest access paths

ruby/ql/lib/codeql/ruby/frameworks/ActionController.qll

@@ -28,7 +28,7 @@ class ActionControllerControllerClass extends ClassDeclaration {
         // In Rails applications `ApplicationController` typically extends `ActionController::Base`, but we
         // treat it separately in case the `ApplicationController` definition is not in the database.
         API::getTopLevelMember("ApplicationController")
-      ].getASubclass*().getAUse().asExpr().getExpr()
+      ].getASubclass().getAUse().asExpr().getExpr()
   }
 
   /**

ruby/ql/lib/codeql/ruby/frameworks/ActiveRecord.qll

@@ -48,7 +48,7 @@ class ActiveRecordModelClass extends ClassDeclaration {
         // In Rails applications `ApplicationRecord` typically extends `ActiveRecord::Base`, but we
         // treat it separately in case the `ApplicationRecord` definition is not in the database.
         API::getTopLevelMember("ApplicationRecord")
-      ].getASubclass*().getAUse().asExpr().getExpr()
+      ].getASubclass().getAUse().asExpr().getExpr()
   }
 
   // Gets the class declaration for this class and all of its super classes

ruby/ql/lib/codeql/ruby/frameworks/GraphQL.qll

@@ -71,7 +71,7 @@ private class GraphqlRelayClassicMutationClass extends ClassDeclaration {
 private class GraphqlSchemaResolverClass extends ClassDeclaration {
   GraphqlSchemaResolverClass() {
     this.getSuperclassExpr() =
-      graphQlSchema().getMember("Resolver").getASubclass*().getAUse().asExpr().getExpr()
+      graphQlSchema().getMember("Resolver").getASubclass().getAUse().asExpr().getExpr()
   }
 }
 
@@ -92,7 +92,7 @@ private class GraphqlSchemaResolverClass extends ClassDeclaration {
 class GraphqlSchemaObjectClass extends ClassDeclaration {
   GraphqlSchemaObjectClass() {
     this.getSuperclassExpr() =
-      graphQlSchema().getMember("Object").getASubclass*().getAUse().asExpr().getExpr()
+      graphQlSchema().getMember("Object").getASubclass().getAUse().asExpr().getExpr()
   }
 
   /** Gets a `GraphqlFieldDefinitionMethodCall` called in this class. */