Using Postgres' copy_from with dynamic table

[lutter]

Hello!

I am trying to use COPY FROM for bulk inserts where the table I want to insert into is only known at runtime. My first thought was that I could do that with a custom implementation of CopyTarget, but its walk_target method doesn't take self, and can therefore only be used for tables known at compile time. My second thought was to implement CopyFromExpression but that trait is not publicly visible.

What's the best advice on doing the above? Also, I haven't looked into it in detail, but I would like to do something similar with COPY TO, i.e., bulk read from a table that is only known at runtime. Any advice for that would also be greatly appreciated.

Thanks a ton in advance!

[weiznich]

I think that's something that's just not supported by the current implementation provided by diesel. We are certainly open to accept PR's adding support there, as long as they don't break any public API. I cannot provide a design for that as I do currently not have the capacity for that.

[lutter]

Thanks for the quick response! At least I now know that I am not missing anything obvious. I'll see if I can come up with a solution/PR myself.

[weiznich]

What might be possible is changing the signature of CopyTarget::walk_ast to take the table as explicit argument. That method is #[doc(hidden)], so it's not part of the public API.

[lutter]

Cool - I didn't realize that that would make CopyTarget::walk_ast not part of the public API.

I tried your suggestion (commit) but got stuck trying to figure out how to get a CopyTarget into the impl CopyFromExpression for InsertableWrapper here I believe if I can get that to work, we should be good.

[weiznich]

There is a table instance in here: https://github.com/diesel-rs/diesel/blob/master/diesel/src/pg/query_builder/copy/copy_from.rs#L375

You likely need to pass it as additional argument down here: https://github.com/diesel-rs/diesel/blob/master/diesel/src/pg/query_builder/copy/copy_from.rs#L527 through the function chain until it reaches the linked walk_ast call.

[lutter]

I came up with something here - I hope that's reasonable