BelongsToMany Instance.add{$plural} sourceKey get() used, but set() on join table not

BelongsToMany association doesn't use custom set on sourceKey field.

I have custom set() and get() on my primary key on a message model. I'm using ulid on a BINARY(16) field. In my API I'm using the canonical form, e.g. '01DNBHWZZ9CMW7MW5Z9NHFNTNF'.

This works fine in and of itself, but as soon as I begin creating associations, the get is fired to get the association key, but the set is not when saving into the association table, or into a foreignKey field, etc.

getUlid and setUlid are functions for converting between binary and hex, and vice versa (UlidMonotonic is imported from npm package id128).

function getUlid( this: any ) {
  // outcoming Buffer -> canonical
  let ulidMonotonic = UlidMonotonic.construct( this.getDataValue('ulid') )
  return ulidMonotonic.toCanonical()
}
function setUlid( this: any, value: Buffer ) {
  //incoming canonical --> Buffer
  let ulidBuffer = UlidMonotonic.fromCanonical( value )
  this.setDataValue('ulid', Buffer.from(ulidBuffer.bytes))
}

Message table:

  Message.init({
      ulid: {
        type: 'BINARY(16)'
      , primaryKey: true
      , get: getUlid
      , set: setUlid
      }
    , message: {
        type: DataTypes.STRING(191).BINARY
      , allowNull: false
      }
    }
  , {
      sequelize
    , tableName
    }
  )
  Message.belongsToMany(User, { as: "receivers", through: "messageReceivers" })

Association table:

  MessageReceiver.init({
      ulid: {
        type: 'BINARY(16)'
      , primaryKey: true
      }
    , messageUlid: {
        type: 'BINARY(16)'
      , references: {
          model: Message
        , key: 'ulid'
        }
      , get: getUlid
      , set: setUlid
      }
    , userUlid: {
        type: 'BINARY(16)'
      , references: {
          model: User
        , key: 'ulid'
        }
      }
    }
  , {
      sequelize
    , tableName
    }
  )

Cut down version of User:

  User.init(
    {
      ulid: {
        type: 'BINARY(16)'
      , primaryKey: true
      }
    , firstName: { type: new DataTypes.STRING(32).BINARY, allowNull: false }
    , lastName: { type: new DataTypes.STRING(32).BINARY, allowNull: false }
    , role: { type: new DataTypes.STRING(32).BINARY, allowNull: false }
    }
  , {
      sequelize
    , tableName
    }
  )

I expect that .addReceivers([user1, user2]) should use the defined set() method on the messageUlid field to store the ulid in binary form on the association table.

Instead, this error occurs because it's trying to insert a 26 character string into a 16 byte binary field.

sqlMessage: "Data too long for column 'MessageUlid' at row 1",                                                                                                  
sql: "INSERT INTO 'messageReceivers' ('createdAt','updatedAt','MessageUlid','UserUlid') VALUES ('2019-09-22 03:58:28','2019-09-22 03:58:28','01DNBHWZZ9CMW7MW5Z9 NHFNTNF',X'016d571e7fe96548b85ff5c30820765d'),('2019-09-22 03:58:28','2019-09-22 03:58:28','01DNBHWZZ9CMW7MW5Z9NHFNTNF',X'016d571e7fe96545959960e350c63e05');"

Environment

• Sequelize version: 5.15.0
• Node.js version: v12.6.0
• Operating System: Archlinux

Issue Template Checklist

How does this problem relate to dialects?

• I think this problem happens regardless of the dialect.
• I think this problem happens only for the following dialect(s):
• I don't know, I was using mariadb, with connector library version XXX and database version XXX

Would you be willing to resolve this issue by submitting a Pull Request?

• Yes, I have the time and I know how to start.
• Yes, I have the time but I don't know how to start, I would need guidance.
• No, I don't have the time, although I believe I could do it if I had the time...
• No, I don't have the time and I wouldn't even know how to start.

Start at the BelongsToMany instance add{$plural} path exercised by .addReceivers([user1, user2]) and trace how sourceKey values are written to the association table. Reproduce the binary(16) ULID case with the shown custom getUlid and setUlid methods; done means the association insert invokes the setter and stores the binary value rather than the 26-character string.