How does odata know which property is the ID? - rest

Question

In the following example russelwhyte is the id, but how does the provider know to map it to the UserName property?
URL: http://services.odata.org/V4/TripPinServiceRW/People?$format=application/json;odata.metadata=full
Response:
{
#odata.context: "http://services.odata.org/V4/(S(ck3fzk3dze0kmzjcruxiz31i))/TripPinServiceRW/$metadata#People",
#odata.nextLink: "http://services.odata.org/V4/TripPinServiceRW/People?%24format=application%2fjson%3bodata.
metadata%3dfull&%24skiptoken=8",
value:
[
{
#odata.type: "#Microsoft.OData.SampleService.Models.TripPin.Person",
#odata.id: "http://services.odata.org/V4/TripPinServiceRW/People('russellwhyte')",
#odata.etag: "W/"08D17DBDFB9CCAAC"",
#odata.editLink: "http://services.odata.org/V4/TripPinServiceRW/People('russellwhyte')",
UserName: "russellwhyte",
FirstName: "Russell",
LastName: "Whyte",
...
Friends#odata.associationLink: "http://services.odata.org/V4/TripPinServiceRW/People('russellwhyte')/Friends/$ref",
Friends#odata.navigationLink: "http://services.odata.org/V4/TripPinServiceRW/People('russellwhyte')/Friends",
...
},
...
]
}

Answer 1

By overriding GetEntityByKey of the base class EntitySetController

Answer 2

Query the metadata document, you can get the model schema for the service, and in which you can find which properties are the keys/IDs.
For example:
GET: http://services.odata.org/V4/(S(ksn5grnrgbebt44osly5z2vr))/TripPinServiceRW/$metadata
<EntityType Name="Person" OpenType="true">
<Key>
<PropertyRef Name="UserName"/>
</Key>
<Property Name="UserName" Type="Edm.String" Nullable="false">
<Annotation Term="Org.OData.Core.V1.Permissions">
<EnumMember>Org.OData.Core.V1.Permission/Read</EnumMember>
</Annotation>
</Property>
<Property Name="FirstName" Type="Edm.String" Nullable="false"/>
<Property Name="LastName" Type="Edm.String" Nullable="false"/>
<Property Name="Emails" Type="Collection(Edm.String)"/>
<Property Name="AddressInfo" Type="Collection(Microsoft.OData.SampleService.Models.TripPin.Location)"/>
<Property Name="Gender" Type="Microsoft.OData.SampleService.Models.TripPin.PersonGender"/>
<Property Name="Concurrency" Type="Edm.Int64" Nullable="false">
<Annotation Term="Org.OData.Core.V1.Computed" Bool="true"/>
</Property>
<NavigationProperty Name="Friends" Type="Collection(Microsoft.OData.SampleService.Models.TripPin.Person)"/>
<NavigationProperty Name="Trips" Type="Collection(Microsoft.OData.SampleService.Models.TripPin.Trip)" ContainsTarget="true"/>
<NavigationProperty Name="Photo" Type="Microsoft.OData.SampleService.Models.TripPin.Photo"/>
</EntityType>