.count

• Return value: integer
• Counts total of instances, or # of values in an array

Order.count

.create

• Return value: New created instance of a class

Order.create(name: "Americano", price: 5.55)

.pluck

• Return value: an array of values grabbed that match the plucked column names
• Extremely helpful when used with other selectors

Person.pluck(:name)
# SELECT people.name FROM people
# => ['David', 'Jeremy', 'Jose']

Person.pluck(:id, :name)
# SELECT people.id, people.name FROM people
# => [[1, 'David'], [2, 'Jeremy'], [3, 'Jose']]

Person.distinct.pluck(:role)
# SELECT DISTINCT role FROM people
# => ['admin', 'member', 'guest']

Person.where(age: 21).limit(5).pluck(:id)
# SELECT people.id FROM people WHERE people.age = 21 LIMIT 5
# => [2, 3]

.sum

• Return value: integer
• Calculates the sum of a column

Person.sum(:age) # => 4562

.maximum & .minimum

• Return value: integer
• Finds the max/min value of a table column

Person.maximum(:age) # => 93
Person.minimum(:age) # => 93

.where

• Return value: Instance(s) of class
• Can take various arguments, including a string, array or hash. A single string is often 1 argument, and an array can be used for multiple arguments, where the first element in the array is treated as a template. It can also accept a hash, with the key/value pairs you're looking for.

String example:
Client.where("orders_count = '2'")

Array example:
User.where(["name = :name and email = :email", { name: "Joe", email: "joe@example.com" }])

Hash example:
User.where({ name: "Joe", email: "joe@example.com" })

.max_by & min_by

• Return value: enumerator
• Good to use in methods, as its return value by itself is not a common wanted value. Looks for either the min/max values

Coffee.all.max_by{ |c| c.orders.size}
Coffee.all.min_by{ |c| c.orders.size}

.first & .last

• Return value: Instance of class
• Very useful in conjunction with .sort

Order.all.sort.last

Sources: rubydoc.info