Today I Learned

hashrocket A Hashrocket project

Ruby memoization with nil values

As Ruby developers, we’re often looking for ways to reduce time consuming lookups in our code. A lot of times, that leads us to memoizing those lookups with the common ||= operator.

However, if our lookups return a nil or falsey value, our memo will actually keep executing the lookup:

def ticket
  @ticket ||= Ticket.find_by(owner:)
end

This code essentially boils down to:

def ticket
  @ticket = @ticket || Ticket.find_by(owner:)
end

If our find_by in the example above returns nil, the code will continue to run the find_by every time we call the ticket method.

To avoid this, we can shift our pattern a bit, and look to see if we have already set our instance variable or not:

def ticket
  return @ticket if defined?(@ticket)
  @ticket = Ticket.find_by(owner:)
end
See More #ruby TILs
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