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6/04/2015

WebAPI OData v.4 and A Double Applied $Skip

6/04/2015 Posted by William Berry , , , , , , No comments
As I noted in my last post, I have been struggling to implement a LINQ provider over a Solr index.  While the adventure has been trying at times, for the most part been, it has been quite successful.  My final hurdle, one that I had been struggling with for the last week or so, was quite confounding ... when the $skip option was used, it was seemingly double applied.  To fully understand this behavior, let's look at a few examples:
  • A query string of ~?$top=10&$skip=10 over a set of 100 results yields 0 results.
  • A query string of ~?$top=10&$skip=8 over a set of 100 results yields 2 results.
Before I explain the crazy, let's take a look at the Controller's Get:


[EnableQuery(
    AllowedQueryOptions = AllowedQueryOptions.Count
                          | AllowedQueryOptions.Filter
                          | AllowedQueryOptions.OrderBy
                          | AllowedQueryOptions.Skip
                          | AllowedQueryOptions.Top)]
[ODataRoute]
public PageResult<Foo> Get(ODataQueryOptions<Foo> queryOptions)
{

    var bars = new QueryableData<Foo>(_provider);

    var result = ((IQueryable<Foo>)queryOptions
        .ApplyTo(bars,
            new ODataQuerySettings(new ODataQuerySettings { EnableConstantParameterization = false, EnsureStableOrdering = false }))).ToList();
    var count = _provider.Count;
    return new PageResult<Foo>(result, null, count);
}

We start off with our Enable Query Attribute to set which Query Options we will allow.  In this case we'll use everything but $select and $expand, and follow that with our OData routing attribute.  The Get method will return a PageResult since we wand to handle the paging and counting and not delegate that responsibility to the framework(s), and lastly, we will also manually handle applying the ODataQueryOptions.

While it is not required to use the overload here with the ODataQueryOptions, it can be a helpful for certain optimizations.  In the event that the count query option was applied the framework will pass the query expression to the queryable twice, once with a return type of 'long and then again with a return type of IQueryable<T>.  Since all Solr queries implicitly return with the num_results property, we can ignore the count expression query and pass the results count back through the query provider thereby saving either two calls to the database or having to implement futures.

Now, back to our problem.

The prudent developer, when challenged with unexpected results from a method call might drop a breakpoint on the return statement and see what values are being returned.  Doing this left me rather puzzled ... the query string ?$top=10&$skip=10 returned a PageResult<Foo>(Foo[10], null, null), exactly what was expected.  I flipped back over to chrome to find 'value:[ ]' on my screen.

After a few hours days of trying different things I elected to load up the GitHub issues list for OData and came across this post: OData PageResult method ignoring count parameter when using EnableQuery attribute #159.  Reading through the issue I had the idea that maybe, just maybe, the EnableQuery attribute was re-applying the query options over the PageResult.  After commenting out the attribute, the method began to function as expected and I had a few fistfuls of hair to spare.

Hope this helps someone!

For internet linking purposes ... here is my stack overflow question and answer.




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