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# Storing SHA1 Hashes As Binary
Storing SHA1 hashes as strings is not very space efficient. A SHA1 as a string
requires at least 40 bytes, an additional byte to store the encoding, and
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perhaps more space depending on the internals of PostgreSQL.
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On the other hand, if one were to store a SHA1 as binary one would only need 20
bytes for the actual SHA1, and 1 or 4 bytes of additional space (again depending
on database internals). This means that in the best case scenario we can reduce
the space usage by 50%.
To make this easier to work with you can include the concern `ShaAttribute` into
a model and define a SHA attribute using the `sha_attribute` class method. For
example:
```ruby
class Commit < ActiveRecord::Base
include ShaAttribute
sha_attribute :sha
end
```
This allows you to use the value of the `sha` attribute as if it were a string,
while storing it as binary. This means that you can do something like this,
without having to worry about converting data to the right binary format:
```ruby
commit = Commit.find_by(sha: '88c60307bd1f215095834f09a1a5cb18701ac8ad')
commit.sha = '971604de4cfa324d91c41650fabc129420c8d1cc'
commit.save
```
There is however one requirement: the column used to store the SHA has _must_ be
a binary type. For Rails this means you need to use the `:binary` type instead
of `:text` or `:string` .
