ruby中do关键字的用法

Question

classBooksdefself.up
create_table:booksdo|t|
t.column:title,:string,:limit=>32,:null=>false
t.column:price,:float
t.column:subject_id,:integer
t.column:description,:text
t.column:created_at,:timestamp
end
end
defself.down
drop_table:books
end
end
create_table:booksdo|t|
这一句没有迭代操作为什么会有rubydo|t|这句呢这里的rubydo|t|到底是什么意思？求高人解答