exist_vs_present.rb

# If we care to know a record exists, but actually don't
# need to do anything with the record...
if User.where(email: "teemo@shrooms.net").present?
  puts "There is a user with email address teemo@shrooms.net"
else
  puts "There is no user with email address teemo@shrooms.net"
end
# then using `present?` is wasteful, as it selects all the column
# and loads the object into memory.
# SELECT * FROM users WHERE email` = 'teemo@shrooms.net';

# Instead, use `exists?`
if User.where(email: "teemo@shrooms.net").exists?
  puts "There is a user with email address teemo@shrooms.net"
else
  puts "There is no user with email address teemo@shrooms.net"
end
# The advantage is the corresponding query limits only to 1 record 
# and does not select any columns.
# SELECT 1 FROM users WHERE email` = 'teemo@shrooms.net' LIMIT 1;