Store OpenERP attachments to MongoDB

To migrate data, rewrite datas field of ir.attachment model like the following erppeek snippet:

ira = C.IrAttachment
ira_ids = ira.search()

for ira_id in ira_ids:
    datas = ira.read(ira_id, ['datas'])['datas']
    ira.write([ira_id], {'datas': datas})