Specify custom function for children order by

Issue Description

Is your feature request related to a problem? Please describe.

I have a model Exercise and I would like to specify a function for order by for children association. For example, I have the following scope defined in Exercise :

// to deal with the horrible order by clauses generically generated
orderByClauses(fields) {
	// To handle order by clauses
	// the last item in the values array will be the choice of the user (instead of the '')
	const ORDER_BY_FIELDS = {
		"state": ["state", ''],
		"id": ["id", ''],
		"title": ["title", ''],
		"date": ['updatedAt', ''],
		"avg_score": [
			{model: sequelize.models.Exercise_Metrics, as: "metrics"},
			'avg_vote_score',
			''
		],
		"vote_count": [
			{model: sequelize.models.Exercise_Metrics, as: "metrics"},
			'vote_count',
			''
		],
	};
	return {
		order: fields.map(field => {
			let order_by_clause = ORDER_BY_FIELDS[field.field];
			// Remove the last empty value so that we can add
			order_by_clause.pop();
			order_by_clause.push(field.value);
			return order_by_clause;
		})
	};
}

Currently, it works as expected but I would like a custom filter on array so I added a new order by clause possibility in ORDER_BY_FIELDS :

{
"tags_count": [
	{model: sequelize.models.Exercise_Metrics, as: "metrics"},
	Sequelize.fn("COALESCE",
		Sequelize.fn(
			"array_length",
			Sequelize.col("tags_ids"),
			1
		),
		0
	),
	''
   ]
}

But Sequelize generates something wrong :

-- ... SOME SELECT STUFF
FROM "exercises_library"."Exercises" AS "Exercise" 
INNER JOIN "exercises_library"."Exercises_Metrics" AS "metrics"
ORDER BY "metrics".COALESCE(array_length("tags_ids", 1), 0) DESC
-- ... REST, LIKE OFFSET OR LIMIT

Describe the solution you’d like

As showcase in docs, we could use a complex object for ordering column

[{model: Task, as: 'Task'}, 'createdAt', 'DESC']

So why not allow something like this ?

const customFn = Sequelize.fn("COALESCE",
		Sequelize.fn(
			"array_length",
			Sequelize.col("tags_ids"),
			1
		),
		0
);
[{model: sequelize.models.Exercise_Metrics, as: "metrics", fn: customFn }, 'DESC']

That way, no breaking change and still backwards compatibility.

Why should this be in Sequelize

It could be useful for order by clauses for many functions : COALESCE, LENGTH , etc …

Describe alternatives/workarounds you’ve considered

• string manipulation with fn : no maintenable in long term
• include properties ( but not possible in many case )

Issue Template Checklist

Is this issue dialect-specific?

• No. This issue is relevant to Sequelize as a whole.
• Yes. This issue only applies to the following dialect(s): XXX, YYY, ZZZ

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.

[papb]

With the current awful codebase of Sequelize

🤣 I have to agree with you on this. Recently I have been trying to refactor a lot of stuff before proceeding with more fixes…

Any thoughts ?

Hmm what comes to my mind right now actually is: would you like to help me refactor stuff around? So that the codebase gets more approachable? That is probably the best path to enable better features in the future

[jy95]

I identified the parts of the code that should be updated to handle this new behaviour

To not prepend model name when we alias the attributes getQueryOrders