In ActiveRecord, what is the difference between the has_many and has_many :through associations?
has_many
has_many :through
Both are one-to-many associations but with has_many :through, the declaring model can associate through a third model.
The has_many: through association is the one-to-many equivalent to the belongs_to one-to-one association.
has_many: through
belongs_to
Both associations are identical, and has_many: through is maintained only for legacy purposes.
The has_many association is a one-to-many association, while has_many: through is a one-to-one association that matches through a third model.