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The PACT documentation clearly states how to select for a single condition in the where clause, but it is not so clear on how to select for multiple clauses which seems much more general and important for real world use cases than a single clause example.

Pact-lang select row function link

For instance I was trying to select a set of dice throws across the room name and the current round.

(select 'throws (where (and ('room "somename") ('round 2)))

But this guy didn't resolve and the error was not so clear. How do I select across multiple conditions in the select function?

KDAbet
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3 Answers3

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The first thing we tried was to simply select via a single clause which returns a list:

(select 'throws (where (and ('room "somename")))

A: [object{throw-schema},object{throw-schema}]

And then we applied the list operator "filter" to the result:

(filter (= 'with-read-function' 1) (select 'throws (where (and ('room "somename"))))

Please keep in mind we had a further function that read the round and spit back the round number and we filtered for equality of the round value.

This ended up working but it felt very janky.

KDAbet
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The second thing we tried was to play around with the and syntax and we eventually found a nice way to express it although not quite so intuitive as we would have liked. It just took a little elbow grease.

The general syntax is:

(select 'throws (and? (where condition1...) (where condition2...))

In this case the and clause was lazy, hence the ? operator. We didn't think that we would have to declare where twice, but its much cleaner than the filter method we first tried.

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The third thing we tried via direction from the Kadena team was a function we had yet to purview: Fold DB.

(let* ((qry (lambda (k obj) true)) ;; (f (lambda(x) [(at 'firstName x), (at 'b x)])) ) (fold-db people (qry) (f)) )

This actually is the most correct answer but it was not obvious from the initial scan and would be near inscrutable for a new user to put together with no pact experience.

We suggest a simple sentence ->

"For multiple conditions use fold-db function."

In the documentation.

This fooled us because we are so used to using SQL syntax that we didn't imagine that there was a nice function like this lying around and we got stuck in our ways trying to figure out conditional logic.

KDAbet
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