code: func (o *InvitationManager) Get(username string, organization string, role string, status InvitationStatus) (*JoinOrganizationInvitation, error) {
  	var orgRequest JoinOrganizationInvitation
  
  	query := bson.M{
  		"user":         username,
  		"role":         role,
  		"organization": organization,
  		"status":       status,
  	}
  
  	err := o.collection.Find(query).One(&orgRequest)
  	if err == mgo.ErrNotFound {
  		return nil, nil
  	}
  
  	return &orgRequest, err
  }
docs: //Get get an invitation by it's content, not really this usefull, TODO: just make an exists method