Alternate keys is supported in Web API OData V5.7. For detail information about alternate keys, please refer to here

The related sample codes can be found here

Enable Alternate key

Users can enable alternate key in the global configuration.

HttpConfiguration config = ...

Model builder

So far, an Edm model with alternate keys can be built by ODL APIs.

EdmEntityType customer = new EdmEntityType("NS", "Customer"); 
customer.AddKeys(customer.AddStructuralProperty("ID", EdmPrimitiveTypeKind.Int32)); 
customer.AddStructuralProperty("Name", EdmPrimitiveTypeKind.String); 
var ssn = customer.AddStructuralProperty("SSN", EdmPrimitiveTypeKind.String); 
model.AddAlternateKeyAnnotation(customer, new Dictionary<string, IEdmProperty> 
    {"SSN", ssn} 

So, SSN is an alternate key.

Routing for alternate key

In OData controller, Users can use the attribute routing to route the alternate key. The Uri template is similiar to function parameter. For example:

public IHttpActionResult GetCustomerBySSN([FromODataUri]string ssn)