Joining more than 3 tables in a row and merging scopes crashes

[SebastianZaha]

As seen in attached gist, I have created 5 tables, connected by belongs_to one to one relationships.

Joining 3 of them in order works fine, joining the 4th one yields an incorrect sql query, where the table join order is wrong, and joining 4 of them raises an ActiveRecord::ConfigurationError

I initially thought it is related to #15461 but I tried the patch and it does not solve this issue, so it might be a separate problem.

[SebastianZaha]

Tests: https://gist.github.com/SebastianZaha/6feeffe2660f2c61362d

[thedarkone]

It is related to #15461: you found another place (in ActiveRecord::Relation::Merger#merge_joins) where AR wasn't preserving JOIN clause order. I'll modify #15461 to fix this bug as well.

As for ActiveRecord::ConfigurationError its tiny bug in ActiveRecord::Relation::Merger#merge_joins, this will get rid of it:

- relation.joins! rest
+ relation.joins! *rest

I will also fix that in #15461.

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