How to save an entity with a collection of related entities using WCF data services

Question

I have a new ParameterPart entity that is related to a new InputOutputMap. The InputOutputMap has several InputStates that I pull from the database and I need to associate with the InputOutputMap. How do I save this chain to the database using MVC and WCF Data services? I am using the following code (being called through an ajax call, context is only being set in the first call in the constructor) but am getting several issues:

• When I use AddLink (as shown below), I am able to add data the first time I try it (irrespective of how many inputstates I have to associate). If however, the method is called again (through ajax), I am only able to add data if I am specifying one inputstate. If I have multiple inputstates, I get context is already tracking the relationship. Note that the method is being called using an ajax call.
• I have tried using SetLink, AddRelatedObject and attach but everytime I am getting errors in the above scenario. Sometimes, the error is that context is already tracking the entity or the relationship. At other times, the context is not tracking the entity.

• I didn't get any benefit when I set the context in the method instead of the constructor.

  if (vm != null)
      {
          ParameterPart parameterPart = null;
  
          // Create a new parameter
          if (vm.PartNumberId == 0)
          {
              // Create an instance of ParameterPart
              parameterPart = new ParameterPart()
              {
                  Description = vm.ParameterDescription                        
              };
  
              // Save the ParameterPart into the database
              try
              {
                  ctx.AddToParameterParts(parameterPart);
                  ctx.SaveChanges();
              }
              catch (System.Exception ex)
              {
                  throw;
              }
          }
          else
          {
              // Fetch the existing parameter
              parameterPart = new ParameterPart();
              parameterPart = (from pp in ctx.ParameterParts
                               where pp.PartNumberId == vm.PartNumberId
                               select pp).Single();
  
              // Update the ParameterPart from the vm
              parameterPart.Description = vm.ParameterDescription;
          }
  
          if (parameterPart != null)
          {
              if (vm.StateValues.Count > 0)
              {
                  InputOutputMap inputOutputMap = new InputOutputMap();
                  inputOutputMap.PartNumberId = parameterPart.PartNumberId;
  
                  ctx.AddToInputOutputMaps(inputOutputMap);
  
                  // Prepare a new InputOutputMap
                  foreach (var state in vm.StateValues)
                  {
                      if (state.InputStateId != 0)
                      {
                          // Fetch the inputstate
                          var inputState = (from i in ctx.InputStates
                                            where i.InputStateId == state.InputStateId
                                            select i).Single();
  
                          try
                          {
                              ctx.AddLink(inputOutputMap, "InputStates", inputState);
                              ctx.SaveChanges();
                          }
                          catch (System.Exception ex)
                          {
                              throw;
                          }
                      }                            
                  }
              }
          }
      }
  

Answer 1

The AddLink method was working as desired. I was getting an error due to a data issue.