Keyed subset of Date to character should "just work"

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Reproduce the keyed Date subset example in the issue and trace the join-subset entry point used by DT[.('2019-01-01')]. Compare it with the working d == '2019-01-01' case, then resolve the proposed Date-to-character coercion and the .Date(0.1) rounding edge case before adding coverage.

Written by the indexing model from the issue text.

Description

IDate/ITime joins 
DT = data.table(d = as.Date('2019-01-01'), key = 'd')

# fails since d is actually numeric
DT[.('2019-01-01')]

# this works since R coerces type for ==
DT[d == '2019-01-01']

Since we're already used to == implicitly coercing dates-as-strings to dates, I think there's reasonable expectation that join subsets would work similarly.

Logic is: if x's join key inherits from Date & i's join key is character, try as.IDate.

Type laxness presents a weird edge case -- what if x has .Date(0.1), should we round it first?

Dominant language
R
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