Added code for defining SHA attributes

These attributes are stored in binary in the database, but exposed as
strings. This allows one to query/create data using plain SHA1 hashes as
Strings, while storing them more efficiently as binary.

2 files changed, 37 insertions, 0 deletions

diff --git a/doc/development/README.md b/doc/development/README.md
index 9496a87d84d..a2a07c37ced 100644
--- a/doc/development/README.md
+++ b/doc/development/README.md
@@ -54,6 +54,7 @@
 - [Polymorphic Associations](polymorphic_associations.md)
 - [Single Table Inheritance](single_table_inheritance.md)
 - [Background Migrations](background_migrations.md)
+- [Storing SHA1 Hashes As Binary](sha1_as_binary.md)
 
 ## i18n
 
diff --git a/doc/development/sha1_as_binary.md b/doc/development/sha1_as_binary.md
new file mode 100644
index 00000000000..3151cc29bbc
--- /dev/null
+++ b/doc/development/sha1_as_binary.md
@@ -0,0 +1,36 @@
+# 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
+perhaps more space depending on the internals of PostgreSQL and MySQL.
+
+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`.