code: @PostMapping("/addEntityMapping")
    public String addEntityMapping(
        @RequestParam String mappingProjectId, String target, String source) {
      EntityType sourceEntityType = dataService.getEntityType(source);
      EntityType targetEntityType = dataService.getEntityType(target);
  
      MappingProject project = mappingService.getMappingProject(mappingProjectId);
      EntityMapping mapping = project.getMappingTarget(target).addSource(sourceEntityType);
      mappingService.updateMappingProject(project);
      autoGenerateAlgorithms(mapping, sourceEntityType, targetEntityType, project);
  
      return "redirect:" + getMappingServiceMenuUrl() + "/mappingproject/" + mappingProjectId;
    }
docs: Adds a new {@link EntityMapping} to an existing {@link MappingTarget}
  
  @param target the name of the {@link MappingTarget}'s entity to add a source entity to
  @param source the name of the source entity of the newly added {@link EntityMapping}
  @param mappingProjectId the ID of the {@link MappingTarget}'s {@link MappingProject}
  @return redirect URL for the mapping project