Previous value for VIRTUAL data returns undefined

[mschipperheyn]

Issue Description

model

{
	id: {
		type: DataTypes.UUID,
		defaultValue: DataTypes.UUIDV4,
		primaryKey: true,
		lowercase: true,
	},
	firstName: {
		type: DataTypes.STRING(255),
	},
	lastName: {
		type: DataTypes.STRING(255),
	},
	displayName: {
		type: DataTypes.VIRTUAL(DataTypes.STRING, [
			'firstName',
			'lastName',
		]),
		get() {
			return `${this.get('firstName') || ''} ${this.get('lastName') ||
				''}`.trim();
		},
	},
},
{
	sequelize,
	hooks: {
		afterUpdate: (item, options) => {
			console.log(item._previousDataValues.displayName); // undefined
			console.log(item.previous('displayName')); // undefined

			console.log(item._previousDataValues.firstName); // value
			console.log(item.previous('firstName')); // value
			
			
		}
	},
},

What are you doing?

updating a record

What do you expect to happen?

I expect VIRTUAL fields to be populated with the actual data

What is actually happening?

previous values are undefined

Additional context

There's obviously a workaround for this, so, I'm registering this also for other people to be aware.

Environment

• Sequelize version: 4.44.3
• Node.js version: 12.13.0
• Operating System: Mac os

Issue Template Checklist

How does this problem relate to dialects?

• [x ] 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 PUT-YOUR-DIALECT-HERE, with connector library version XXX and
  database version Mysql 5.7

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.
• [x ] 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.

[coolhand79]

I'm interested to know what the known workaround for this is... simply not to use them?

[github-actions]

This issue has been automatically marked as stale because it has been open for 7 days without activity. It will be closed if no further activity occurs. If this is still an issue, just leave a comment or remove the "stale" label. 🙂

[blapaz]

This still appears to be an issue in v6

[ephys]

VIRTUAL types are basically just getters & setters, they don't have an entry in dataValues or previousDataValues

Which makes me wonder if we should not deprecate VIRTUAL altogether, and recommend using native getter & setters instead