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💡 TL;DR - Specify expression / function that must be true, else exception

Constraints declare an expression (lambda) or function that must be true for a transaction to complete. You also provide a message that is returned in the exception that is raised if the expression is not true.

Provide expression as lambda

The simplest contraint is shown below. The first rule is a constraint using a lambda. Note the argument is the row, providing access to the attributes.

You can stop in the debuggers and examine values when the lambda is invoked.


Provide function - old_row, verb

For more complex cases, you can provide a function. Note the arguments include old_row and logic_row. The latter provides access to the verb, so you can make your logic apply only to the desired verbs.



Commit constraints - aggregate values

If your constraint needs to reference derived sums or counts, note these are not derived when the row is initially processed. You can address such situations using CommitConstraints.