如何在rails postgresql数据库中创建BLOB列

Question

I'm attempting to store binary data in a database. (postgresql on heroku)

I understand there are two different ways to store binary data in postgresql. A blob and a bytea..

When I create a table in my migration,

create_table :binaries do |t|
  t.binary :data
end

it creates a column in the database of type bytea.

My question is.. How do I create a record of type blob?

Why do I ask? It seems when I send a ten byte file up to heroku, it stores it as a string of hex values, prepended with an "e".. so my 10 bytes becomes 21. My 10 meg file would become 20 megs (and one byte), ext, ext, ext...
Now that bothers me, but as I don't really care about performance. (I've had the care beaten out of me by the PM), its not what bothers me the most.
What really bothers me is; when I read out the contents of the database I get the 21 bytes, not the 10. That is un-useable.

So my question again.. How do I create a BLOB column in rails/postgresql/heroku environment?

Answer 1

bytea is PostgreSQL's version of a BLOB. From the fine manual :

The SQL standard defines a different binary string type, called BLOB or BINARY LARGE OBJECT . The input format is different from bytea , but the provided functions and operators are mostly the same.

So bytea is what you want. As far as the format goes:

The bytea type supports two external formats for input and output: PostgreSQL's historical "escape" format, and "hex" format. Both of these are always accepted on input. The output format depends on the configuration parameter bytea_output; the default is hex. (Note that the hex format was introduced in PostgreSQL 9.0; earlier versions and some tools don't understand it.)

So what you're seeing is just the text versions that are used for getting data into the database and out of the database.

This might also be of interest:

• ActiveRecord loads binary field incorrectly on Heroku, fine on OSX

Answer 2

Blob storage is built into Postgres and is supported using the ActiveRecord adapter, but only directly through the raw connection and the lo_* methods. You can use lo_write, lo_open, lo_close and lo_read to create and manipulate blobs. Creating a blob returns an OID which you can reference the blob in your models.

You can add this using a migration

rails g migration AddFileToModel file:oid

Or directly like this

add_column :users, :avatar, :oid

For a working example of this you should look at the Carrierwave PostgreSQL gem . You can either build a custom solution based on that code, or just use Carrierwave directly. I'm currently using it and it works well.