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TADDataSet.Constraints Property
TADDataSet Class  TADDataSet Members  uADCompDataSet Namespace  Example  public  See Also  Send Feedback

Specifies record-level client constraints that must be met when editing the data.

Group

public

Links
TADDataSet Class, TADDataSet Members, uADCompDataSet Namespace, Example, public, See Also
Description

Use Constraints to maintain record-level constraints for the dataset. 

Record-level constraints force business ruless limiting few fields in a single record. This constraints are checked at end of data editing (Post / AppendRecord / InsertRecord). Constraints, limiting single field are field constraints and must specified at TField.CustomConstraint or TField.ImportedConstraint. 

The Constraints checking is performed when ConstraintsEnabled is True. If application needs to perform large updates to the dataset and application can guarantee the data consistancy, then set ConstraintsEnabled to False before updates, and return to original value after updates. The constraints are not checked at data fetching. 

The expressions in Constraints must be predicates, evaluating to Boolean value. AnyDAC supports expression syntax compatible with:

  • BDE data access components;
  • Client dataset;
  • Oracle 8 (not for 100%).

See Writing Expressions for details of how to write constraint expressions.

Syntax
property Constraints;
See Also

ConstraintsEnabled, DisableConstraints, EnableConstraints, BeginBatch, EndBatch, TDataSet.Post, TDataSet.AppendRecord, TDataSet.InsertRecord, TField.CustomConstraint, TField.ImportedConstraint

Example
with ADMemTable1.Constraints.Add do begin
  CustomConstraint := 'sal + bonus <= 2000';
  ErrorMessage := 'The employee payments must be equal or less than 2000 usd';
end;

ADMemTable1.ConstraintsEnabled := True;
ADMemTable1.Edit;
try
  ADMemTable1.FieldByName('sal').AsFloat := 1800;
  ADMemTable1.FieldByName('bonus').AsFloat := 300;
  // here exception will be raised
  ADMemTable1.Post;
except
  ADMemTable1.Cancel;
  Application.HandleException(Self);
end;

ADMemTable1.ConstraintsEnabled := False;
ADMemTable1.Edit;
ADMemTable1.FieldByName('sal').AsFloat := 1800;
ADMemTable1.FieldByName('bonus').AsFloat := 300;
// here exception will be NOT raised, because constraint checking is disabled
ADMemTable1.Post;
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