Table: key
Fields:
    id - Unique identifier for each key.
    opens - id of device the key opens.  We're assuming a many-to-one
            relationship but that could be many to many by adding another
            table mapping which targets share the same key.
    available - 0 or 1.
    date_created
    checkout_date
    user_id
    due_date

##</code><code>##

Table: key_history
    id - unique history id.
    key_id - maps to the id of the key.
    action - created, checked-out, checked-in, destroyed.
    date_added
    user_id