Proper way to call nested Expand() from .NET OData 4 Client

问题

Problem

In an OData 4 service on Web API, what is the proper way to call nested $expand from a .NET client? We are using the OData Client Generator. Back in the prior WCF Data Services with OData 3 service we could call .Expand("Customers/Orders"). In Web API with OData 4 we can no longer do so and receive the following should you attempt .Expand("Customers/Orders"):

The query specified in the URI is not valid. Found a path traversing multiple navigation >properties. Please rephrase the query such that each expand path contains only type >segments and navigation properties.

Workaround

We are able to work around this by calling expand like so: .Expand("Customers($expand=Orders)"). In non-nested $expand scenarios, I like the lambda support like so .Expand(d => d.Customers). Is there a proper way in .NET OData 4 client to call nested expands without the unfortunate magic string of .Expand("Customers($expand=Orders)")? If not, is there a cleaner string pattern like "Customers/Orders" that would work? Thanks.

回答1:

In OData v4, it is not valid to expand multi levels, such as what you mentioned in the question: .Expand("Customers/Orders"). I dont think the client will support such API. Here is what in the ABNF http://docs.oasis-open.org/odata/odata/v4.0/os/abnf/odata-abnf-construction-rules.txt:

expand            = '$expand' EQ expandItem *( COMMA expandItem )
expandItem        = STAR [ ref / OPEN levels CLOSE ]
                  / expandPath
                    [ ref   [ OPEN expandRefOption   *( SEMI expandRefOption   ) CLOSE ]
                    / count [ OPEN expandCountOption *( SEMI expandCountOption ) CLOSE ]
                    /         OPEN expandOption      *( SEMI expandOption      ) CLOSE 
                    ]
expandPath        = [ qualifiedEntityTypeName "/" ] 
                    *( ( complexProperty / complexColProperty ) "/" [ qualifiedComplexTypeName "/" ] )
                    navigationProperty 
                    [ "/" qualifiedEntityTypeName ]

回答2:

The request that you want to send is:

GET http://host/service/Customers/Orders

right?

According to the OData protocol:

To request related entities according to a particular relationship, the client issues a GET request to the source entity’s request URL, followed by a forward slash and the name of the navigation property representing the relationship.

So such request is not supported as the "Customers" before "/Orders" is the name of an entity set instead of a single entity. You can only write nested expanding like:

GET http://host/service/Customers(1)/Orders

Which corresponds to the following code snippets using OData V4 Code Generator:

var orders = context.Customers.ByKey(new Dictionary<string, object>() { { "ID", 1 } }).Orders.Execute();

And you need to iterate through all the customers to get all their orders.

来源：https://stackoverflow.com/questions/25246865/proper-way-to-call-nested-expand-from-net-odata-4-client