FEAT: `dj.Top` orders the preview with `order_by`

[CBroz1]

Feature Request

Problem

I'm using dj.Top to monitor the top results of a given computation. Intuitively, running MyTable & dj.Top(order_by='field DESC', limit=n) should show the top n values of this field, in this order. Instead, I need to scan the result ordered by primary key, or fetch it.

Requirements

The script below outlines expected vs actual outcome.

Demo

import datajoint as dj

schema = dj.schema("cbroz_temp")


@schema
class TopDemo(dj.Lookup):
    definition = """
    id : int
    ---
    my_var: varchar(64)
    my_float: float
    """

    contents = [(1, "C", 2.0), (2, "D", 4.0), (3, "A", 1.0), (4, "B", 3.0)]


if __name__ == "__main__":
    print("TopDemo contents:\n", TopDemo())
    print(
        "Order by my_float ascending:\n",
        "Expected IDs 3, 4, 1. Got IDs: 1, 3, 4\n",
        TopDemo & dj.Top(order_by="my_float ASC", limit=3),
    )
    print(
        "Order by my_var descending:\n",
        "Expected IDs 2, 1, 4. Got IDs: 1, 2, 4\n",
        TopDemo & dj.Top(order_by="my_var DESC", limit=3),
    )

Justification

This would cut down on redundant use of order_by

Alternative Considerations

Currently, I pass the same order_by arg to a fetch with format='frame'

(MyTable & dj.Top(order_by="field DESC", limit=3)).fetch(order_by"field DESC", format=Frame)

Related Errors

n/a

Please include steps to reproduce provided errors as follows:

• OS: Linux
• Python Version: 3.9.6
• MySQL Version: 8 latest
• MySQL Deployment Strategy: local docker
• DataJoint Version: 0.14.3
• Minimum number of steps to reliably reproduce the issue: see above
• Complete error stack as a result of evaluating the above steps: n/a

Screenshots

n/a

Additional Research and Context

preview is already running the alternative step of fetching as a frame

datajoint-python/datajoint/preview.py

Lines 6 to 13 in 5f37f83

	def preview(query_expression, limit, width):
	heading = query_expression.heading
	rel = query_expression.proj(*heading.non_blobs)
	if limit is None:
	limit = config["display.limit"]
	if width is None:
	width = config["display.width"]
	tuples = rel.fetch(limit=limit + 1, format="array")

If query expressions were assigned an order_by attr by Top, QueryExpression.preview could pass this as a kwarg to preview.