Question

I am trying to store a long text (in my case a raw rss feed, but could just as well be a long blog post or similar) to a MySql database.

I have a migration with:

change_column :contents, :description, :longtext

But this gives a schema.rb with:

t.text     "description",       :limit => 2147483647

When the limit should in fact have been set to 4294967295.

Why does Rails impose an upper limit which is half of what should be possible?

Answer 1

I don’t know if rails supported :longtext officially in previous versions, but in the current version according to rails documentation, :longtext is in fact not specified as legal datatype. I think it’s only a convenience of the mysql adapter that translates this to :text.

The correct way to do it is then:

change_column :contents, :description, :text, :limit => 4294967295

Keep it mind that the effective maximum size is less when using multi-byte characters.

Edit: Thinking a second about it and it makes sense that rails halves the size. Re-reading the mysql docs on this topic , they speak of effective size. I guess specifying 2147483647 could lead to an effective size of 4294967295 when filled with 2-byte UTF-8 characters. As UTF-8 is the default encoding in ruby 1.9 (at least on my machine), it’s the only correct way to do it. I think?!

ruby-1.9.2-p136 :002 > "".encoding
=> #<Encoding:UTF-8>