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Tip 44 – How to navigate an OData compliant service

  • Artigo

I recently did a crash course in Data Services and OData.

While doing so I realized my notes might be useful for you guys.

So here is my little cheat sheet to quickly get up to speed with OData Urls.

Note: OData Services may not necessarily support all of the following features: but if they do, this is how you would use them.

The Service:

It all starts with a Data Service hosted somewhere:

https://server/service.svc

Basic queries:

You access the Data Service entities through resource sets, like this:

https://server/service.svc/People

You request a specific entity using its key like this:

https://server/service.svc/People(16)

Or by using a reference relationship to something else you know:

https://server/service.svc/People(16)/Mother

This asks for person 16’s mother.

Once you have identified an entity you can refer to it’s properties directly:

https://server/service.svc/People(16)/Mother/Firstname

$value:

But the last query wraps the property value in XML, if you want just the raw property value you append $value to the url like this:

https://server/service.svc/People(16)/Mother/Firstname/$value

$filter:

You can filter resource sets using $filter:

https://server/service.svc/People?$filter=Firstname  eq ‘Fred’

Notice that strings in the filter are single quoted.

Numbers need no quotes though:

https://server/service.svc/Posts?$filter=AuthorId eq 1

To filter by date you have identity the date in the filter, like this:

https://server/service.svc/Posts?$filter=CreatedDate eq DateTime'2009-10-31'

You can filter via reference relationships:

https://server/service.svc/People?$filter=Mother/Firstname eq 'Wendy'

The basic operators you can use in a filter are:

Operator

Description

C# equivalent

eq

equals

==

ne

not equal

!=

gt

greater than

>

ge

greater than or equal

>=

lt

less than

<

le

less than or equal

<=

and

and

&&

or

or

||

()

grouping

()

There are also a series of functions that you can use in your filters if needed.

$expand:

If you want to include related items in the results you use $expand like this:

https://server/service.svc/Blogs?$expand=Posts

This returns the matching Blogs and each Blog’s posts.

$select:

Some Data Services allow you to limit the results to just the properties you require – aka projection – for example if you just want the Id and Title of matching Posts you would need something like this:

https://server/service.svc/Posts?$select=Id,Title

You can even project properties of related objects too, like this:

https://server/service.svc/Posts?$expand=Blog&$select=Id,Title,Blog/Name

This projects just the Id, Title and the Name of the Blog for each Post.

$count:

If you just want to know how many records would be returned, without retrieving them you need $count:

https://server/service.svc/Blogs/$count

Notice that $count becomes one of the segments of the URL – it is not part of the query string – so if you want to combine it with another operation like $filter you have to specify $count first, like this:

https://server/service.svc/Posts/$count?$filter=AuthorId eq 6

This query returns the number of posts authored by person 6.

$orderby:

If you need your results ordered you can use $orderby:

https://server/service.svc/Blogs?$orderby=Name

Which returns the results in ascending order, to do descending order you need:

https://server/service.svc/Blogs?$orderby=Name%20desc

To filter by first by one property and then by another you need:

https://server/service.svc/People?$orderby=Surname,Firstname

Which you can combine with desc if necessary.

$top:

If you want just the first 10 items you use $top like this:

https://server/service.svc/People?$top=10

$skip:

If you are only interested in certain page of date, you need $top and $skip together:

https://server/service.svc/People?$top=10&$skip=20

This tells the Data Service to skip the first 20 matches and return the next 10. Useful if you need to display the 3rd page of results when there are 10 items per page.

Note: It is often a good idea to combine $top & $skip with $orderby too, to guarantee the order results are retrieved from the underlying data source is consistent.

$inlinecount & $skiptoken:

Using $top and $skip allows the client to control paging.

But the server also needs a way to control paging – to minimize workload need to service both naive and malicious clients – the OData protocol supports this via Server Driven Paging.

With Server Driven Paging turned on the client might ask for every record, but they will only be given one page of results.

This as you can imagine can make life a little tricky for client application developers.

If the client needs to know how many results there really are, they can append the $inlinecount option to the query, like this:

https://server/service.svc/People?$inlinecount=allpages

The results will include a total count ‘inline’, and a url generated by the server to get the next page of results.

This generated url includes a $skiptoken, that is the equivalent of a cursor or bookmark, that instructs the server where to resume:

https://server/service.svc/People?$skiptoken=4

Sometime you just need to get the urls for entities related to a particular entity, which is where $links comes in:

https://server/service.svc/Blogs(1)/$links/Posts

This tells the Data Service to return links – aka urls – for all the Posts related to Blog 1.

$metadata

If you need to know what model an OData compliant Data Service exposes, you can do this by going to the root of the service and appending $metadata like this:

https://server/service.svc/$metadata

This should return an EDMX file containing the conceptual model (aka EDM) exposed by the Data Service.

Wrap-up

This should help you get started.

I’ll be exploring Data Services and OData a lot more, and I’ll share with you what I learn, so stay tuned for more.

Comments

  • Anonymous
    November 22, 2009
    Omg!Alex you rock!!! so much useful content in one blog post. I found the content really useful since I never investigated data service. Thanks for putting together the content.
  • Anonymous
    November 22, 2009
    @Zeeshan,Thanks for the kind words.Makes me feel like doing it all over again :)Alex
  • Anonymous
    November 23, 2009
    Hi Alex,I've tried $inlinecount with the data at   http://ogdi.cloudapp.net/v1/dc but it doesn't seem to work. Is this normal or am I missing something?Fabrice
  • Anonymous
    November 24, 2009
    Fabrice,I'm not 100% sure but my guess is OGDI just doesn't support 1.5 features like inline count and projection yet.Alex
  • Anonymous
    November 24, 2009
    Thanks for the reply.You are correct. I can see that from the namespace used in the metadata (http://schemas.microsoft.com/ado/2006/04/edm), vs. the namespace for 1.5 (http://schemas.microsoft.com/ado/2008/09/edm).
  • Anonymous
    March 25, 2010
    Fantastic piece of information, as other posts in your blog.Next one to read: out to build a data-service!
  • Anonymous
    October 24, 2010
    Very informative articlethanks for making this effort