It's a bold statement, but this question proves that many advanced query features have not been fully or properly implemented in the published service.
While many developers may use it to practise OData query concepts, given that the OData implementation is largely up to the developers and the version of the packages they use, it is probably a of more commercial value if you query against a live or a dev implementation of the actual service that you want to query.
The following is an analysis of OPs queries and how to achieve the desired response according the the OData-V4 specification and verified against a deployed API that utilises the following NuGet packages:
- Microsoft.AspNet.OData v7.3.0
- Microsoft.OData.Core v7.6.2
The actual API that I used for testing is proprietary and cannot be published here.
According to the spec, you have to move the $top specifier to the Order_Details expansion.
https://services.odata.org/Experimental/Northwind/Northwind.svc/Customers?
$expand=Orders($expand=Order_Details($top=1))
However:
https://services.odata.org/Experimental/Northwind does not correctly implement $top query option within expansion as defined in 5.1.3 System Query Option $expand
Query options can be applied to an expanded navigation property by appending a semicolon-separated list of query options, enclosed in parentheses, to the navigation property name. Allowed system query options are $filter, $select, $orderby, $skip, $top, $count, $search, and $expand.
$top is however supported in the ODataLib v7+ (.Net) implementation of OData v4. So the syntax is correct, but you should check with your API developers for their opinion if your queries with this syntax do not work.
NOTE: When using $top you should also use $orderbyto ensure that the query results are reliable and reproducable:
https://services.odata.org/Experimental/Northwind/Northwind.svc/Customers?
$expand=Orders($expand=Order_Details($orderby=ProductID;$top=1))
To apply multiple query options within an expansion, you must separate then with a semi-colon: ;. However that alone will not prevent other customer records from being returned, so you must also add a root level filter as well, which is complicated by the fact that Orders is a collection. We can use the any function to only return customers that have an order with the specified Id:
Also note that OrderID is numeric, so do not wrap comparison values in quotes
https://services.odata.org/Experimental/Northwind/Northwind.svc/Customers?
$expand=Orders($expand=Order_Details;$filter=OrderID eq 10643)
&$filter=Orders/any(o:o/OrderID eq 10643)
this can be further simplified using parameter alias:
https://services.odata.org/Experimental/Northwind/Northwind.svc/Customers?
$expand=Orders($expand=Order_Details;$filter=OrderID eq @orderId)
&$filter=Orders/any(o:o/OrderID eq @orderId)
&@orderId='10643'
However:
https://services.odata.org/Experimental/Northwind does not correctly implement parameter aliases so you cannot verify the alias syntax against this service.
Also note
that experimental service is not correctly applying the filter to either the root elements or the navigation collection, the syntax shown here does however work against the .Net ODataLib implementations of OData v4.
The reason that your $expand is not working is that you have left off the $ from the parameter name. The OData query interpreter only identifies query options are parameters that start with $.
Eitherway, according to the https://services.odata.org/Experimental/Northwind/Northwind.svc/$metadata#Regions, there is no Order_Details navigation property to $expand on:
<EntitySet Name="Regions" EntityType="NorthwindModel.Region">
<NavigationPropertyBinding Path="Territories" Target="Territories" />
</EntitySet>
So when you try again with the correct syntax:
https://services.odata.org/Experimental/Northwind/Northwind.svc/Regions?$expand=Order_Details
you get the expected message:
Could not find a property named 'OrderID' on type 'NorthwindModel.Region'
The second attempt will work if you put the correct $ in there for the $expand query option:
https://services.odata.org/Experimental/Northwind/Northwind.svc/Regions?$expand=Territories
The OData query parser only looks for the expected query options with the $ prefix, this allows your API logic to still process other non-OData parameters as you see fit to do so. The other parameters therefor are still HTTP Url compliant parameters, the implementation at odata.org doesn't know what to do with them and they are simply ignored.
This is just another variation on the same issue with 3, the $ is missing. (I suspect that this URL was meant to be in 3: https://services.odata.org/Experimental/Northwind/Northwind.svc/Products?$expand=Suppliers)
So while https://services.odata.org/Experimental/Northwind is not 100% reliable, neither are the .Net ODataLib, SAP or MS Dynamics implementations. The spec is evolving and there are many query techniques that are not fully implemented in probably any providers at this stage.
Simply be mindful of this fact and when you run into issues using an API, the first point of contact should be the developers or the community that are supporting that particular API, it will be up to the developers what techniques and packages they use and at the end of the day to what extent they support the protocol as it is specified.
