TIL - Weird behavior of pluck and uniq in ActiveRecord
Normally, when fetching raw columns data from database in ActiveRecord,
pluck is prefered to map because it only fetches the necessary columns and
does not create any ActiveRecord objects:
# fetch all columns and create activerecord objects
Product.all.map(&:name)
# fetch only necessary columns and create activerecord objects
Product.select(:name).map(&:name)
# fetch only necessary columns and create only necessary primitive objects
Product.pluck(:name)
However, when using with uniq and non-top-level scope, it can catch us offguard:
Product.uniq.pluck(:name) # works, only unique names are returned
category.products.uniq.pluck(:name) # does not work, all names are returned
In that case, we can use uniq after the pluck, but it may cost us a lot of
memory to handle all the returned data:
category.products.pluck(:name).uniq # works, but waste a lot of memory to store redundant data
A faster solution is using DISTINCT keyword directly in pluck, but it may
harm readability/maintainability:
category.products.pluck('DISTICT name') # works, but may fail if name is an aliased column
I wonder why there is this weird behavior…