add bulk upsert function

[Betula-L]

No description provided.

[bombsimon]

This looks like an awful lot of duplication. I think it would be better to create one method that takes an bulk action (INSERT, INSERT IGNORE, INSERT ... ON DUPLICATE KEY etc) and then just pass that query to the bulk function. By doing this there will be a lot of places to keep track of and update when needed. You can have a look at this implementation as a reference.

_{Sorry for shameless self promotion, although not saying you should switch library, just linking as an example.}

[bombsimon]

I don't think this is a good way forward for this library, I would rather see keeping the signature but restructuring the code in a way that each query type uses the same code to generate columns and values.

[bombsimon]

Also, don't spend too much time on this, according to https://github.com/jinzhu/gorm/issues/255#issuecomment-590287329 this bulk feature will be featured in Gorm v2 which will be released soon.

[Betula-L]

OK. Thanks.

[bombsimon]

@Betula-L Didn't mean you should close this! The hint about the coming feature was just so you don't feel you spend unnecessary time but I'll gladly accept the feature in this project!

I think the feature you added is great, I just thing a different approach would be better to improve maintainability and keep it more DRY.

Please feel free to reopen this issue and I'll gladly help you out with tips and/or ideas!