If I do User.all.pluck(:email) then it works fine.
But if I do
arr = Array.new
arr = User.all
and then
arr.pluck(:email)
this is raising following error
undefined method `pluck' for #<Array:0x007f4ff8daf3c8>
which means I cannot use pluck with arrays, so how can we get particular field value from an array of records in just one line like above. I don't want to loop through each record in an array.
pluck is useful to do a minimalist db query.
When you have an array, just use map:
arr.map(&:email)
pluck ...
arr = User.all ; arr.map with User.pluck is far more optimal. His question is ambiguous with regards to whether altering the initial selection from the DB is possible. I just think the performance improvement of getting the DB to provide what you need rather than manipulating the data after fetching it is worth considering.Use collect, it's an Array method:
arr.collect{|u| u.email}
pluck(:x) is the equivalent of select(:x).map(&:x) on an ActiveRecord collection.
If you have an array, Array#map and its alias Array#collect do the same job.
If you use
User.scoped.pluck(:email)
your query will be like
SELECT users.email FROM users
So, to answer the question, you can NOT use pluck on an array, pluck is an ActiveRecord::Calculations method, not an array one.
pluck causes a modification to the SELECT portion of the DB query. The map method iterates over an Array. Whilst the products might be the same, the processes to provide them are absolutely NOT equivalent.
select chained by map is equivalent, not map alone.
pluck is the equivalent of select and map together, as @xlembouras does. He wasn't saying it was equivalent to either of them on their own.Converting to array this way is a memory hogger. Instead you can use:
arr = User.scoped
arr.pluck :email
or in a more easy to read:
User.scoped.pluck :email
This will make sure that actual user objects are not loaded into memory until they are required.
User.all.pluck(:email)works fine? what rails version are you using?