Proposal: Creating has_one association example

[davydovanton]

Now we have only has_many association example. I think it will be good to add has_one example too.
WDYT?

/cc @hanami/documentation @jodosha @cllns

[mereghost]

@davydovanton it's not implemented right now and even has_many is still experimental.

[AlfonsoUceda]

@mereghost as far as I know we have implemented all associations isn't it? I think we should create guides for belongs_to, has_one and many_to_many.

[mereghost]

Yep. has_one, has_many, belongs_to and has_many :through (aka many-to-many) working good as far as I made the code bend and stretch itself.