OData Expression

OData is a REST protocol with a standard query language built into the URL — you shape the response with $-prefixed options like $select, $filter and $expand instead of asking the backend for a new endpoint. This page explains those options and shows how to send each one through monoFetchOdata, live against a public OData v4 service.

What is OData

A plain REST endpoint returns whatever the backend decided to return. An OData endpoint returns whatever you ask for, because the query is part of the URL:

GET https://services.odata.org/TripPinRESTierService/People
    ?$select=UserName,FirstName
    &$filter=contains(FirstName,'A')
    &$orderby=FirstName
    &$top=5

Three things come with that:

• One endpoint, many shapes. Fewer fields, filtered rows, sorted, paged — no backend change.
• A published schema. Every service exposes $metadata describing its entities. That is what OData Types generates TypeScript from.
• The work happens on the server. Filtering and aggregation run in the database, not in the browser.

The $-prefixed parts are called system query options. They are the whole subject of this page.

System Query Options

monoFetchOdata gives you two ways to send them:

Channel	What you write	What happens
options	The DevExtreme shape — select, expand, filter, sort, pageSize, pageIndex, requireTotalCount	DevExtreme translates it into the $ params. You never write $ keys here.
params	A raw query-string map — { $apply: '…' }	Sent through verbatim. The only way to reach options DevExtreme has no equivalent for.

The full list:

Option	What it does	Example
$select	Return only these fields	$select=UserName,FirstName
$expand	Inline a related entity	$expand=Trips
$filter	Keep only matching rows	$filter=Age gt 30
$orderby	Sort the result	$orderby=LastName desc
$top	Take at most N rows	$top=10
$skip	Skip the first N rows	$skip=20
$count	Include the total row count	$count=true
$search	Free-text search	$search=Boise
$apply	Group and aggregate server-side	$apply=groupby((Gender),aggregate($count as Total))
$compute	Add a calculated field to the result	$compute=Budget mul 2 as Doubled
$format	Response format	$format=json
$index	Insert position in a collection	$index=0
$schemaversion	Pin the schema version	$schemaversion=1.0

Nested options need params

options.expand only takes plain navigation names. To put inner options inside an expand — $expand=Trips($select=Name;$top=1) — write it as a raw param instead:

params: { $expand: 'Trips($select=Name,Budget;$top=1)' }

Paging is pageSize / pageIndex, not take / skip

monoFetchOdata builds a DevExtreme DataSource, and a DataSource pages itself: pageSize becomes $top and pageIndex becomes $skip (the offset is pageIndex * pageSize). A take or skip passed in options is ignored, and you silently get the DataSource default $top=20 instead.

$apply needs paginate: false

When you send $apply, also set options.paginate = false. Otherwise DevExtreme attaches its default $top=20 and silently truncates the aggregation buckets.

For the operators and functions you can use inside $filter (eq, gt, contains, startswith, any/all, …), see the reference in Mock API.

Try it

Each tab is a real request against the public TripPin OData v4 service. Switch tabs to change the example, then press Run.

The snippet in each tab is not a mock-up — it is the exact monoFetchOdata call the button executes, rendered from the same argument object that gets passed in. After a run the panel also shows the actual request URL the browser sent, read back from the browser's own resource timings, so you can see precisely which $ params came out the other end.

See also

• Data Fetching — configuring fetching.api, configBaseUrl, and auth tokens.
• OData Types — generating TypeScript types from $metadata.
• DataSource — monoCreateFetcher and binding a live DataSource to a component.
• Mock API — the supported $filter operators and the $apply pipeline.