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Ruby on Rails migrations are used to create the necessary tables inside our database. One of the most frequently used column type is string.
Here is an example of an ActiveRecord::Migration that uses this type:
As you can see, the string is translated to varchar(255). This is the type that allows to store strings with maximum length 255. What if we want to increase the maximum length?
How can we do that? In that case, you need to use the limit: option. Let’s say that we want the biography column to have maximum length 1024. We have to change the corresponding
line in the above migration as follows:
If you do this change and run your migration, the table that is created is the following:
As you can see, we have managed to change the maximum number of characters that biography column can hold. Please, note that the maximum length for a varchar column in
MySQL is 65535theoretically. In practice, this is limited by the maximum row size (65535 bytes which is shared among all columns of the table) and the character set
used. So, for utf-8 database collation, this might be quite smaller.
Perfect. And what if we want to store a text that is longer than 65535 characters long? What is the option that we have?
If you try to set a bigger limit, you will probably get an error. The next option is to use the migration type text.
If you change your migration for biography as follows:
As you can see, the text migration type has been translated to text MySQL type. And the question now is: Is that bigger than varchar(65535)? The answer is no. MySQL
documentation mentions that text maximum length is 216.
So, if you really want to store larger texts than that, you need to consider the two MySQL types: mediumtext and longtext. These can store really huge amounts of text.
mediumtext can store up to 224 and longtext up to 232.
The problem here is that if, for example, you want the biography to be a longtext, this:
will not work. Trying to run this migration will give you the following error:
undefined method `longtext' for #<ActiveRecord::ConnectionAdapters::TableDefinition:0x...db/migrate/..._create_authors.rb:5:in `block in change'
This is because longtext is not considered a standard migration column type. Neither mediumtext does.
The alternative here is to use the column method instead, and pass the column type as 2nd argument:
This will run successfully and will generate the table:
2) If you use the limit: option you can change the default 255 to whatever.
3) Note that text and character varying(n) in PostgreSQL are the same except from the fact that character varying(n) puts a limit. text does not have one.
4) You cannot use mediumtext or longtext with PostgreSQL. text does the job for you.
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