feat(count): initial implementation of loadCount

[AzariasB]

Instead of populating a collection to get the number of items, fetches the count directly from the database. (collection.loadCount())
An optional parameter refresh (false by default) can be passed to the function to force a database query instead of using the cache.

I still need to update the documentation.

Any feedback would be greatly appreciated :)

[B4nan]

please remove all code style changes.

[AzariasB]

Ow, didn't see all the changes made by the vscode plugin :|, will fix that

[B4nan]

left some comments, but in general its looking good, ty!

[AzariasB]

I'm having an issue that has been fixed for #956 when doing tests for n:m relations (Cannot read property '__meta' of undefined), should I merge mikro-orm master's branch into my own ?

[B4nan]

Yes, you need to rebase and resolve conflicts. Or close it and create new PR if you struggle with that, and just pick the changes you made.

[AzariasB]

So, createCondition was incorrect for n:m relations as it just returned a condition dictionary with empty ids: {id : {$in : [] } }
I'm not sure what I did is the correct way of creating the condition though ...

[B4nan]

createCondition works fine, the collection items need to be set upfront to use it. in other words, it is not the method you should be using for this use case.

take a look at loadFromPivotTable, that is where this happens (but it loads the items - their PKs - so its again not what you should use, but a method for inspiration).

[AzariasB]

Are there existing entities with composite primary keys and many-to-many relationships so that I can test this case ?

[B4nan]

https://github.com/mikro-orm/mikro-orm/blob/master/tests/composite-keys.sqlite.test.ts
https://github.com/mikro-orm/mikro-orm/blob/master/tests/composite-keys.mysql.test.ts

[AzariasB]

I'm having an issue when testing the loadCount with many-to-many relationships :
With composite keys, the count queries looks like : select count(distinct `e0`.`first_name`, `e0`.`last_name`) as count ... which is not a valid SQL query (too many arguments for count). I'm not sure how to deal with this ?

[B4nan]

That's a bug in knex, sqlite dialect does not support distinct count with multiple fields. I'd say ignore that for now, maybe comment that test out and point to this issue as this is blocked by knex:

#889

In the long run I am planning to remove the dependency on knex and build the queries myself to have absolute control, there are more and more issue popping up that are hard or impossible to work around...

[B4nan]

There are few minor things I'd do differently, but let's not clutter this up :]

Last thing worth testing is how this works with mongo, as the m:n collections are implemented differently (no pivot tables).

Also there is still one brach not tested in Collection.ts:270.

[AzariasB]

Yes, I still need to add more tests (mysql composite keys)
For mongo, is there an existing test file for composite keys ?

[B4nan]

There are no composite keys in mongo, don't worry about that.

[lgtm-com]

This pull request introduces 1 alert when merging e8dec6f into 732b45b - view on LGTM.com

new alerts:

• 1 for Unused variable, import, function or class

[AzariasB]

I'm having an issue with the count method of the entitymanager for mongo-db :
The tests fails when using count with a filter. Am I using the filters incorrectly ?

[B4nan]

There are no joins in mongo, it is not possible to query anything from the inverse side, so yes, this is correct behaviour in mongo unfortunately.

Also M:N collections are implemented as arrays on the owning entity, so the actual count on the entity will be always the correct one. You should make it conditional in a similar way to the Collection.init() (based on reference type and Platform.usesPivotTable method).

[AzariasB]

All right, I think everything's good now :)

[B4nan]

Ok, let's merge it, thanks!

[AzariasB]

Perfect, thanks !
It was a bit tougher than I expected
Thank a lot for the help throughout this pull request :), and thank you very much for the work you put into mikro-orm !