private IQueryable <TEntity> BuildGetQuery(ISpecification <TEntity> spec)
{
if (spec == null)
{
throw new ArgumentNullException(nameof(spec));
}
var query = DbContext.Set <TEntity>().AsQueryable();
query = spec.AddPredicates(query);
query = spec.AddSorting(query);
query = spec.AddEagerFetching(query);
return(query);
}