Create a many_through_many association. Arguments:

• name - Same as associate, the name of the association.
• through - The tables and keys to join between the current table and the associated table. Must be an array, with elements that are either 3 element arrays, or hashes with keys :table, :left, and :right. The required entries in the array/hash are:

  :table (first array element) :	The name of the table to join.
  :left (middle array element) :	The key joining the table to the previous table. Can use an array of symbols for a composite key association.
  :right (last array element) :	The key joining the table to the next table. Can use an array of symbols for a composite key association.

  If a hash is provided, the following keys are respected when using eager_graph:

  :block :	A proc to use as the block argument to join.
  :conditions :	Extra conditions to add to the JOIN ON clause. Must be a hash or array of two pairs.
  :join_type :	The join type to use for the join, defaults to :left_outer.
  :only_conditions :	Conditions to use for the join instead of the ones specified by the keys.

• opts - The options for the associaion. Takes the same options as many_to_many.

[Source]

     # File lib/sequel/plugins/many_through_many.rb, line 161
161:         def many_through_many(name, through, opts={}, &block)
162:           associate(:many_through_many, name, opts.merge(through.is_a?(Hash) ? through : {:through=>through}), &block)
163:         end