Set the DataSet Relations

hi all, I have a dataset with 5 tables: tb_Projects(pk_ProjectID,name,...) tb_Donors(pk_DonnorID,name,...) tb_Clients(pk_ClientID,code,name,...) tb_ProjectDonors(pk_ProjectDonorID,fk_projectID,fk_DonnorID) tb_ProjectClients(pk_ProjectClientID,fk_ClientID,fk_ProjectID). And the code, i set the Relation as below: ds.Relations.Add( "ProjectProjectDonors", ds.Tables["Projects"].Columns["pk_ProjectID"], ds.Tables["ProjectDonors"].Columns["fk_ProjectID"]); ds.Relations.Add( "ProjectDonors", ds.Tables["ProjectDonors"].Columns["fk_DonnorID"], ds.Tables["Donors"].Columns["pk_DonnorID"]); ds.Relations.Add( "ProjectProjectClients", ds.Tables["Projects"].Columns["pk_ProjectID"], ds.Tables["ProjectClients"].Columns["fk_ProjectID"]); ds.Relations.Add( "ProjectClients", ds.Tables["ProjectClients"].Columns["fk_ClientID"], ds.Tables["Clients"].Columns["pk_ClientID"]); But in the runtime the system shows error "This constraint cannot be enabled as not all values have corresponding parent values" at the row : ds.Relations.Add( "ProjectClients", ds.Tables["ProjectClients"].Columns["fk_ClientID"], ds.Tables["Clients"].Columns["pk_ClientID"]); Please help me to fix this. Thanks, Tin Nguyen