async function getValue(key, serviceId = null, userId = null) {
  const variable = await db.Variable.findOne({
    where: {
      name: key,
      service_id: serviceId,
      user_id: userId,
    },
  });

  // if variable was not found
  if (!variable) {
    return null;
  }

  // else, return the value
  return variable.value;
}
@description Get the value of a variable.
@param {string} key - The unique key of the variable.
@param {string} [serviceId] - The unique ID of a service, or null.
@param {string} [userId] - The unique ID of a user, or null.
@returns {Promise} - Resolve with the variable.
@example
variable.getValue('API_KEY', '5bbaaea4-2ad6-4f3e-9bbc-819b9d310309', 'd1d73559-a987-44eb-9453-3cbf5bcb5a2f');