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Propagating constraints from middle-tier to client

Question: How do I propagate constraints from Middle-tier to Client? Answer: This example uses the demos found in \Delphi 3\Demos\DB\CLIENTDS\Empedit.dpr and \server.dpr. Empedit.dpr represents the client tier, server.dpr runs on the middle tier, and the database, DBDEMOS, is located also on the middle tier, but in a real world scenario would be an SQL database running on yet a third machine. This example uses the Employee.db table. In order to use the following, you will need to set up DCOM and have the two examples running before adding the complication of constraints to the mix. The general steps are: Define constraints in the data dictionary Associate the data dictionary attributes to a TField Access the TField from a ClientDataSet. First, define an attribute set in the data dictionary that includes a constraint. For instance, an attribute set called EmpNoA could have a CustomConstraint of "EmpNo > 100" and a ConstraintErrorMessage of "EmpNo is less than 100". Define static TFields in Server.dpr. Bring up the EmpServer datamodule and double click EmpQuery to bring up the fields editor. In the fields editor, right-click the EmpNo TField, select Associate Attributes, and select EmpNoA from the Associate Attributes dialog. Now when selecting the EmpNo TField in the fields editor you should see CustomConstraint and ConstraintErrorMessage matching the data dictionary entry. Open EmpEdit.dpr and add a TDBEdit to EmployeeForm. Set the DataField property to EmpNo and the DataSource property to EmpData (the TClientDataSet). Compile and run Server.dpr on the middle tier machine, then on the client machine run the EmpEdit example application. Attempt to post a record where EmpNo is greater than 100; your custom error message will appear.