How does `sort_by` work with multiple fields?

Question

In the following piece of code, due_date and position are optional fields, whereas important is a field which is always set to either true or false.

posts.sort_by do |post| 
  [post.due_date || 0, post.position || 0, post.important? ? 0 : 1]
end

I'm trying to figure out how multiple sort works. If due_date is present, then it takes precedence or sets it to zero. Why are we something setting it to zero here?

If you sort_by a collection words by word.length it compares word lengths. If you compare arrays, IIRC it walks the array until there's something !=, e.g., if the due_date isn't equal, that's the comparison. If it is, it's position, etc. You set something to zero as a way of providing a default value if there's nothing in that field. — Dave Newton
– Dave Newton, Commented Feb 28, 2014 at 21:04
arrays sort by the natural ordering of the first element; if there is a tie, those ties are sorted by the second element; etc. — roippi
– roippi, Commented Feb 28, 2014 at 21:05
Refer to this answer, that's not exactly your problem, but the solution may be of some help. — toro2k
– toro2k, Commented Feb 28, 2014 at 21:28

sawa · Accepted Answer · 2014-02-28 21:36:32Z

1

The problem here is nothing particularly about multiple comparison. If you compare arrays, each of the elements will be compared to the corresponding one in another array. If those values sometimes take numerical values, then they always have to be numerals. Numerals cannot be compared to nil, and doing so will raise an error. Defaulting them to zero is to ensure they are numerals. As long as you default them to a numeral, it will not raise an error. The particular choice of zero was based on where you want to position the entries with the missing values; you could have chosen infinity, negative infinity, etc for different results.

edited Feb 28, 2014 at 21:36

answered Feb 28, 2014 at 21:30

sawa

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4 Comments

123 Over a year ago

how should I handle datetime fields in that case. I cannot default them to zero or Time.now

sawa Over a year ago

You can create a dummy Time object with the Unix epoch.

123 Over a year ago

How do I create that object, and what value would it be equal to.

sawa Over a year ago

Time.new(0). It would be smaller than any other time object.

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