couchDB sorting complex key

Question

I have a couchDB database which has several different document "types" which all relate to a main "type".

In the common blog / post example, the main type is the blog post, and the others are comments (though there are 3 different types of comments.

All of the types have a date on them, however, I wish to sort blog posts by date, but return all of the data from the comments as well. I can write an emit which produces keys like so:

[date, postID, docTypeNumber]

where docTypeNumber is 1 for post and > 1 for the different comment document types.

e.g:

["2013-03-01", 101, 1]
[null, 101, 2]
[null, 101, 2]
[null, 101, 3]
["2013-03-02", 101, 1]
[null, 102, 2]
[null, 102, 3]

Of course, If I emit this, all the nulls get sorted together. Is there a way to ignore the nulls, and group them by the seccond item in the array, but sort them by the first if it is not null?

Or, do I have to get all the documents to record the post date in order for sort to work?

I do not want to use lists, they are way too slow and I'm dealing with a potentially large data set.

David V David V · Accepted Answer · 2013-03-06T23:49:56

You can do this by using conditionals in your map function.

if(date != null) {
  emit([date, postID, docTypeNumber]);
}
else {
  emit([postID, docTypeNumber]);
}

I don't know if you want your array length to be variable or not. If not, you could add the sort variable first. The following snippet could work since date and postID presumably never have the same values.

if(date != null) {
  sortValue = date;
}
else {
  sortValue = postID;
}
emit(sortValue, date, postID, docTypeNumber);

Update: I thought about this a little more. In general, I make my views based on queries I want to perform. So I ask myself, what do I need to query? It seems that in your case, you might have two distinct queries here. If so, I suggest having two different views. There is a performance penalty to pay since you would run two views instead of one, but I doubt it is perceivable to the user. And it might take up more disk space. The benefit for you would be clearer and more explicit code.

couchDB sorting complex key

2 Answers