meh.rb

class Reader
  has_many :memberships
  has_many :magazines, through: :memberships
end

class Membership
  belongs_to :reader
  belongs_to :magazine
end

class Magazine
  has_many :memberships
  has_many :readers, through: :memberships
end

# What's the easiest way to get the `Membership` for
# a given Reader and Magazine?

@reader.memberships.find_by_magazine_id(@magazine.id)

# Or is there a better way?

Nice solution. Personally I prefer association with :extend option as more structural way

class Magazine
  module FinderExtension
    def of_reader(reader)
      find_by_reader_id(reader.id)
    end
  end

  has_many :memberships, extend: FinderExtension
end