DPDP02_g6.cs

public interface IOpportunityRepository : IRepository<Opportunity>
{
    List<Opportunity> GetByAccountId(Guid id);
}
public class OpportunityRepository : RepositoryBase<Opportunity>, IOpportunityRepository
{
    public OpportunityRepository(IOrganizationService service) : base(service)
    {
    }

    public List<Opportunity> GetByAccountId(Guid id)
    {
        var query = this.ServiceContext.OpportunitySet
            .Where(o => o.CustomerId != null && o.CustomerId.Id == id);
        return query.ToList<Opportunity>();
    }

}