code: function(m1, m2, fship) {
      return this.findOneAndUpdate({
        _id: m1,
        [pathName]: { $elemMatch: { _id: m2 } },
      }, {
        $set: {
          [`${pathName}.$.status`]: fship.status,
        },
      }, { new: false })
        .then(() => fship);
    }
docs: Update friendship
  
  @returns a function to update a friendship between two parties
  @api private