In
the previous post, you saw how to create an OData feed and pre-filter
the data. In this post, we will see how to shape
the data. A sample project is attached at
the bottom of this post. Pre-filtering and shaping OData feeds using WCF Data Services and
the Entity Framework - Part 1 Shaping
the feed
The Product feed we created earlier returns too much information about our products. Let’s change this so that only
the following properties are returned – ProductID, ProductName, QuantityPerUnit, UnitPrice, UnitsInStock. We also want to return only Products that are not discontinued. Splitting
the Entity To shape our data according to
the requirements
above, we are going to split our Product Entity into two and expose one through
the feed.
The exposed entity will contain only
the properties listed
above. We will use
the other Entity in our Query Interceptor to pre-filter
the data so that discontinued products are not returned. Go to
the design surface for
the Entity Model and make a copy of
the Product entity. A “Product1” Entity gets created. Rename Product1 to ProductDetail. Right click on
the Product entity and select “Add Association” Make a one to one association between Product and ProductDetails. Keep only
the properties we wish to expose on
the Product entity and delete all other properties on it (see diagram below). You delete a property on an Entity by right clicking on
the property and selecting “delete”. Keep
the ProductID on
the ProductDetail. Delete any other property on
the ProductDetail entity that is already present in
the Product entity. Your design surface should look like below: Mapping Entity to Database Tables Right click on “ProductDetail” and go to “Table Mapping” Add a mapping to
the “Products” table in
the Mapping Details. After mapping ProductDetail, you should see
the following. Add a referential constraint. Lets add a referential constraint which is similar to a referential integrity constraint in SQL. Double click on
the Association between
the Entities and add
the constraint with “Principal” set to “Product”. Let us review what we did so far. We made a copy of
the Product entity and called it ProductDetail We created a one to one association between these entities Excluding
the ProductID, we made sure properties were not duplicated between these entities We added a ProductDetail entity to Products table mapping (Entity to Database). We added a referential constraint between
the entities. Lets build our project. We get
the following error: ”'NortwindODataFeed.Product' does not contain a definition for 'Discontinued' and no extension method 'Discontinued' accepting a first argument of type 'NortwindODataFeed.Product' could be found …"
The reason for this error is because our Product Entity no longer has a “Discontinued” property. We “moved” it to
the ProductDetail entity since we want our Product Entity to contain only properties that will be exposed by our feed. Since we have a one to one association between
the entities, we can easily rewrite our Query Interceptor like so: [QueryInterceptor("Products")]
public Expression<Func<Product, bool>> OnReadProducts()
{
return o => o.ProductDetail.Discontinued == false;
}
Similarly, all “hidden” properties of
the Product table are available to us internally (through
the ProductDetail Entity) for any additional logic we wish to implement.
Compile
the project and view
the feed. We see that
the feed returns only
the properties that were part of
the requirement.
To see
the data in JSON format, you have to create a request with
the following request header
Accept: application/json, text/javascript, */* (easy to do in jQuery)
The result should look like this:
{
"d" : {
"results": [
{
"__metadata": {
"uri": "http://localhost.:2576/DataService.svc/Products(1)", "type": "NorthwindModel.Product"
}, "ProductID": 1, "ProductName": "Chai", "QuantityPerUnit": "10 boxes x 20 bags", "UnitPrice": "18.0000", "UnitsInStock": 39
}, {
"__metadata": {
"uri": "http://localhost.:2576/DataService.svc/Products(2)", "type": "NorthwindModel.Product"
}, "ProductID": 2, "ProductName": "Chang", "QuantityPerUnit": "24 - 12 oz bottles", "UnitPrice": "19.0000", "UnitsInStock": 17
}, {
...
...
If anyone has
the $format operation working, please post a comment. It was not working for me at
the time of writing this.
We have successfully pre-filtered our data to expose only products that have not been discontinued and shaped our data so that only certain properties of
the Entity are exposed. Note that there are several other ways you could implement this like creating a QueryView, Stored Procedure or DefiningQuery.
You have seen how easy it is to create an OData feed, shape
the data and pre-filter it by hardly writing any code of your own.
For more details on OData, Google it with your favorite search engine :-) Also check out
the one of
the most passionate persons I have ever met, Pablo Castro –
the Architect of Aristoria WCF Data Services. Watch his MIX 2010 presentation titled “OData: There's a Feed for That” here.
Download Sample Project for VS 2010 RTM
NortwindODataFeed.zip
