BreezeJs: Error: Unable to convert this endpoint to an IQueryable

Question

I have asp.net core 1.1.0 project and trying the model of CodeCamp sample. In this we a controller which return Lookup data as below :

[BreezeController]
public class BreezeController : ApiController
{

[HttpGet]

public object Lookups()
{
    var rooms = _repository.Rooms;
    var tracks = _repository.Tracks;
    var timeslots = _repository.TimeSlots;
    return new { rooms, tracks, timeslots };
}

And the above Lookups is called in dataContext.js as below:

function getLookups() {
    return EntityQuery.from('Lookups')
        .using(manager).execute()
        .to$q(querySucceeded, _queryFailed);

    function querySucceeded(data) {
        log('Retrieved [Lookups]', data, true);
        return true;
    }
}

Now, I am trying to follow same as above in my project its giving me error as below :

Get http://Localhost:12345//breeze/demo/Lookups 500(Internal server error) Uncaught (in promise)

Error: Unable to convert this endpoint to an IQueryable

Any solution to above issue...its working fine in John Papa's Code camper project. My web api lookups code is working fine if I run it in browser but not with breezejs.

Answer 1

Breeze's .NET Core implementation expects a hidden first parameter. It uses this to perform the IQueryable filtering of a REST operation. For example, if you have an operation that looks like this:

[HttpGet]
public IQueryable<Order> Orders()

And you wanted to get all Orders with the Status of 123 that Cost less than $10 then the first parameter would be something like this:

{ 
   "where":{ 
      "Status":123,      
      "Cost":{ 
         "lt":10
      }
   },
   "select":[ 
      "OrderId"
   ]
}

This is a significant departure from the previous version. The client can be changed to pass parameters compatable with this by adding:

breeze.config.initializeAdapterInstance("uriBuilder", "json");

I added this to my fetchMetadata call.

However, this causes a lot of problems if you have specific get methods with parameters and you want to call it from Swagger or another application.

Something Like this:

[HttpGet]
public IQueryable<Order> GetOrdersByStatusAndLessThanCost(int status, int cost)

Will generate a url like this:

GetOrdersByStatusAndLessThanCost?status=123&cost=10

Breeze assumes that the first parameter (status=123) is its JSON. So it tries to parse it out.

This gives the first most common error with migrating Breeze to .NET Core:

This EntityQuery ctor requires a valid json string. The following is not json: status=123

If you happened to pass in Json, but the result is not an IQueryable, then you will get this error:

Unable to convert this endpoint to an IQueryable

The key to all of this is to give breeze what it is looking for. For the example above the following URL would work:

GetOrdersByStatusAndLessThanCost?{}&status=123&cost=10

Note the added {}& as the first parameter. This tells breeze that there is not anything expected as far as filtering goes.

To get this working for Swashbuckle (and by extension Swagger\Open API) add this to your Startup.cs ConfigureServices method inside the call to services.AddSwaggerGen(c =>:

c.OperationFilter<AddBreezeParameter>();

And then create the file that is needed for that:

public class AddBreezeParameter : IOperationFilter
{
    public void Apply(OpenApiOperation operation, OperationFilterContext context)
    {
        if (context.MethodInfo.ReturnType.Name.StartsWith("IQueryable"))
        {
            if (operation.Parameters == null)
            {
                operation.Parameters = new List<OpenApiParameter>();
            }

            var exampleString = "<br>\":{}," +
                                "<br>&nbsp;&nbsp;&nbsp;&nbsp;\"where\":{" +
                                "<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;\"Status\":123," +
                                "<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;\"Cost\":{" +
                                "<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;\"lt\":10" +
                                "<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}" +
                                "<br>&nbsp;&nbsp;&nbsp;}," +
                                "<br>&nbsp;&nbsp;&nbsp;\"select\":[" +
                                "<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;\"OrderId\"," +
                                "<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;\"OrderDateTime\"" +
                                "<br>&nbsp;&nbsp;&nbsp;]" +
                                "<br>}";


            var breezeJsonParam = new OpenApiParameter
            {
                Name = "{\"breezeJson",
                In = ParameterLocation.Query,
                AllowEmptyValue = true,
                Description =
                    "Json used to query a REST resource.  <br>Due to Breeze's nonstandardness and Swashbuckle's not able to customize to allow for it, this MUST start with \":{} and end with }   In between those you can put your query if it is appropriate.  If you do you must add a comma after the starting value and before you value.  Here is an example: " +
                    exampleString,
                AllowReserved = true
            };
            var schema = new OpenApiSchema {Type = "json", Default = new OpenApiString("\":{}}")};
            breezeJsonParam.Schema = schema;
            operation.Parameters.Insert(0, breezeJsonParam);
        }
        else
        {
            if (operation.Parameters == null)
            {
                operation.Parameters = new List<OpenApiParameter>();
            }

            var breezeJsonParam = new OpenApiParameter();
            // Breeze looks for the first parameter so it can do an IQueryable Filter on it.
            // We want it to not have anything for that parameter if it is not an IQueryable.
            breezeJsonParam.Name = "{}&";
            breezeJsonParam.In = ParameterLocation.Query;
            breezeJsonParam.Description = "Do NOT modify this parameter.  (It is here for Breeze compatibility.)";
            var schema = new OpenApiSchema {Example = new OpenApiString(" ")};
            //var schema = new OpenApiSchema {Type = "string", Default = new OpenApiString("\":{}}")};
            breezeJsonParam.Schema = schema;
            operation.Parameters.Insert(0, breezeJsonParam);
        }

    }
}