[Ur] solution to the DB chicken and egg problem?

[Marko Schütz Schmuck]

On Mon, 03 Apr 2017 09:57:30 -0400,
Adam Chlipala wrote:
> 
> I'm not familiar with a standard database feature to allow that mode, but it sounds like you are referencing
> an extra annotation on constraints.  Somehow this situation has only come up once in the 10+ years of Ur/
> Web's existence, and it's never been an issue for my own Ur/Web apps.

AFAIK "DEFERRABLE INITIALLY DEFERRED" is standard SQL, but I agree
that it might not be of much practical relevance.

What about the other thought: explicitly merging two dml statements
into one transaction (of course stripping the transactions around the
individual dml statements)?

Best regards,

Marko


> On 04/02/2017 10:10 AM, Marko Schütz Schmuck wrote:
> 
>     On Sun, 02 Apr 2017 09:02:15 -0400,
>     Adam Chlipala wrote:
>     
>         Actually, Ur/Web won't even accept those table definitions: no mutually recursive
>         definitions yet, w.r.t. constraints.  And I haven't thought before about allowing
>         temporary breaking of constraints.
>         
>     "Constraints hold after every statement" -> "Constraints hold after
>     every transaction"?
> 
>         On 04/02/2017 08:35 AM, Marko Schütz Schmuck wrote:
>         
>             If I have tables
>             
>             table chicken : { Id : int, Egg : int }
>                    PRIMARY KEY (Id),
>                    CONSTRAINT egg FOREIGN KEY Egg REFERENCES egg(Id)
>             
>             table egg : { Id : int, Chicken : int }
>                    PRIMARY KEY (Id),
>                    CONSTRAINT chicken FOREIGN KEY Chicken REFERENCES chicken(Id)
>             
>             Outside of Ur/Web this would be solved by setting the constraints
>             DEFERRABLE INITIALLY DEFERRED and do the INSERTs pairwise inside a
>             single transaction.
>             
>             I doubt that there currently is a way to do this in Ur/Web since there
>             is neither a way to specify DEFERRABLE INITIALLY DEFERRED nor a way to
>             explicitly form transactions.
>             
>             Would it be enough to introduce an operator
>             
>             dml_sequence : dml -> dml -> dml
>             
>             allowing two dml statements to be composed and run in a single
>             transaction together with exposing DEFERRABLE INITIALLY DEFERRED at
>             the Ur/Web language level?
>             
>             Best regards,
>             
>             Marko
>             
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