What's the difference between find, find_by, and where?

find and find_by return a single record, while where returns a collection of every match, even when nothing matches.

• find searches by primary key and raises ActiveRecord::RecordNotFound if the record is missing.
• find_by takes any conditions and returns the first match, or nil if there is none. Its bang version, find_by!, raises instead.
• where returns an ActiveRecord::Relation. The query does not run until you use the records, so you can chain more conditions first. When nothing matches, you get an empty relation, not nil.

User.find(1)                    # Raises error if not found
User.find_by(email: "a@b.com")  # Returns nil if not found
User.where(active: true)        # Returns Relation (chainable)

Use find when the record must exist, find_by when it may not, and where when you need a set of records.