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Handling Constraints from the Server

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When a database server defines constraints on what data is valid, it is useful if the client dataset knows about them. That way, the client dataset can ensure that user edits never violate those server constraints. As a result, such violations are never passed to the database server where they would be rejected. This means fewer updates generate error conditions during the updating process.

Regardless of the source of data, you can duplicate such server constraints by explicitly adding them to the client dataset. This process is described in Constraining Data Values.

It is more convenient, however, if the server constraints are automatically included in data packets. Then you need not explicitly specify default expressions and constraints, and the client dataset changes the values it enforces when the server constraints change. By default, this is exactly what happens: if the source dataset is aware of server constraints, the provider automatically includes them in data packets and the client dataset enforces them when the user posts edits to the change log.

Note: Only datasets that use the BDE can import constraints from the server. This means that server constraints are only included in data packets when using TBDEClientDataSet or TClientDataSet with a provider that represents a BDE-based dataset. For more information on how to import server constraints and how to prevent a provider from including them in data packets, see Handling Server Constraints.
Note: For more information on working with the constraints once they have been imported, see Using Server Constraints.

While importing server constraints and expressions is an extremely valuable feature that helps an application preserve data integrity, there may be times when it needs to disable constraints on a temporary basis. For example, if a server constraint is based on the current maximum value of a field, but the client dataset uses incremental fetching, the current maximum value for a field in the client dataset may differ from the maximum value on the database server, and constraints may be invoked differently. In another case, if a client dataset applies a filter to records when constraints are enabled, the filter may interfere in unintended ways with constraint conditions. In each of these cases, an application may disable constraint-checking.

To disable constraints temporarily, call the DisableConstraints method. Each time DisableConstraints is called, a reference count is incremented. While the reference count is greater than zero, constraints are not enforced on the client dataset.

To reenable constraints for the client dataset, call the dataset's EnableConstraints method. Each call to EnableConstraints decrements the reference count. When the reference count is zero, constraints are enabled again.

Tip: Always call DisableConstraints and EnableConstraints in paired blocks to ensure that constraints are enabled when you intend them to be.

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