/// <summary>
/// Gets the single resource json:api relationships object by resource identifier lookup.
/// </summary>
/// <typeparam name="TResource">Type of CLR resource.</typeparam>
/// <typeparam name="TResourceId">Type of CLR resource identifier.</typeparam>
/// <param name="clrResourceId">CLR resource identifier value to lookup the single json:api relationships object with.</param>
/// <returns>Returns the single json:api relationships object if found, null otherwise.</returns>
/// <exception cref="DocumentReadException">Is thrown if there are
/// multiple CLR resources with the same resource identifier for the given CLR resource type.</exception>
public static Relationships GetResourceRelationships <TResource, TResourceId>(this IDocumentReader documentReader, TResourceId clrResourceId)
where TResource : class, IResource
{
Contract.Requires(documentReader != null);
var clrResourceType = typeof(TResource);
var apiResourceRelationships = documentReader.GetResourceRelationships(clrResourceType, clrResourceId);
return(apiResourceRelationships);
}