OData = Hotness
Microsoft has officially announced OData. If you are not aware of what this is, then in a sentence: OData is a queryable REST based interface that exposes your data via AtomPub.
To publish a feed, you have to use .Net. However, they have provided client sdk’s for a variety of languages to allow for simple querying of the exposed services and they are working on several more.
I strongly encourage you to check it out.
Partly to give an idea of what is possible, and partly for my own reference, I am going to repost a “cheat sheet” that I found online at Meta-Me
The Service:
It all starts with a Data Service hosted somewhere:
http://server/service.svc
Basic queries:
You access the Data Service entities through resource sets, like this:
http://server/service.svc/People
You request a specific entity using its key like this:
http://server/service.svc/People(16)
Or by using a reference relationship to something else you know:
http://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:
http://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 $value to the url like this:
http://server/service.svc/People(16)/Mother/Firstname/$value
```
###### $filter:
You can filter resource sets using `$filter`:
```
http://server/service.svc/People?$filter=Firstname eq ‘Fred’
```
Notice that strings in the filter are single quoted.
Numbers need no quotes though:
```
http://server/service.svc/Posts?$filter=AuthorId eq 1
```
To filter by date you have identity the date in the filter, like this:
```
http://server/service.svc/Posts?$filter=CreatedDate eq DateTime’2009-10-31′
```
You can filter via reference relationships:
```
http://server/service.svc/People?$filter=Mother/Firstname eq ‘Wendy’
```
The basic operators you can use in a filter are:
<table class="table table-striped"><tbody><thead><tr><th>Operator</th><th>Description</th><th>C# equivalent</th></tr></thead></tbody><tbody><tr><td>eq</td><td>**eq**uals</td><td>==</td></tr><tr><td>ne</td><td>**n**ot **e**qual</td><td>!=</td></tr><tr><td>gt</td><td>**g**reater **t**han</td><td>></td></tr><tr><td>ge</td><td>**g**reater than or **e**qual</td><td>>=</td></tr><tr><td>lt</td><td>**l**ess **t**han</td><td><</td></tr><tr><td>le</td><td>**l**ess than or **e**qual</td><td><=</td></tr><tr><td>and</td><td>and</td><td>&&</td></tr><tr><td>or
</td><td>or</td><td>||</td></tr><tr><td>()
</td><td>grouping</td><td>()</td></tr></tbody></table>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 $expand like this:
```
http://server/service.svc/Blogs?$expand=Posts
```
This returns the matching Blogs and each Blog’s posts.
###### $select:
Some Data Services allow you to limit the results to just the properties you require – aka projection – for example if you just want the Id and Title of matching Posts you would need something like this:
```
http://server/service.svc/Posts?$select=Id,Title
```
You can even project properties of related objects too, like this:
```
http://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`:
```
http://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:
```
http://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`:
```
http://server/service.svc/Blogs?$orderby=Name
```
Which returns the results in ascending order, to do descending order you need:
```
http://server/service.svc/Blogs?$orderby=Name%20desc
```
To filter by first by one property and then by another you need:
```
http://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:
```
http://server/service.svc/People?$top=10
```
###### $skip:
If you are only interested in certain page of date, you need `$top` and `$skip` together:
```
http://server/service.svc/People?$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 `$skip` allows the client to control paging.
But the server also needs a way to control paging – to minimize workload need to service both naive and malicious clients – the OData protocol supports this via [Server Driven Paging](http://blogs.msdn.com/astoriateam/archive/2009/03/19/ado-net-data-services-v1-5-ctp1-server-driven-paging.aspx).
With Server Driven Paging turned on the client might ask for every record, but they will only be given one page of results.
This as you can imagine can make life a little tricky for client application developers.
If the client needs to know how many results there really are, they can append the `$inlinecount` option to the query, like this:
```
http://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 $skiptoken, that is the equivalent of a cursor or bookmark, that instructs the server where to resume:
http://server/service.svc/People?$skiptoken=4
$links
Sometime you just need to get the urls for entities related to a particular entity, which is where $links comes in:
http://server/service.svc/Blogs(1)/$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 $metadata like this:
http://server/service.svc/$metadata
This should return an EDMX file containing the conceptual model (aka EDM) exposed by the Data Service.