select_pluck.rb

# Suppose we load all user records, instantiate them as `User` objects
# only to select one attribute. This is wasteful since we select all
# columns from the database and load them all into memory.
user_emails = User.where(status: "active").map(&:email)
# SELECT * FROM users;

# Instead, we can use `pluck` to modify the query to select only the 
# column(s) we need. It also returns the attributes as an array of strings 
# instead of `User` objects, reducing memory use.
user_emails = User.where(status: "active").pluck(:email)
# Select users.email FROM users;

# If an `ActiveRecord_Relation` of `User` objects is still needed,
# then use `select`.  This returns the result as an `ActiveRecord_Relation` 
# of `User` objects but with only the requested attribute(s) in each object.
user_emails = User.where(status: "active").select(:email)
# Select users.email FROM users;

# Only if you really need ALL the attributes of a model, should you not use
# `pluck` or `select`.
users = User.where(status: "active")