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From Michael Fair <>
Subject Re: Candidates for built-in filter functions?
Date Tue, 22 Mar 2016 21:37:18 GMT
What about some list operation functions?
like "[one of the things in this list] is a member of [the things in that

One of the use cases I often see, and have wanted myself is per document
user authorization.  Without trying to solve everything all at once, being
able to efficiently compare membership multiple lists as a filter would be
generically useful.

Imagine the _users document contained "authTokens" which is simply a list
of strings  or numbers that the user is authorized as.

These authTokens would most likely represent group names that the user
belonged to.  The list is flat, not hierarchical.  If a user is a member of
Group 1, and Group 1 is a member of Group 2; then the authTokens list will
have both Group 1 and Group 2 in the list. (databasename_authTokens would
contain a database specific list of tokens.)

How the list gets assigned is outside the scope here (let's just say
there's either a management app or the app itself has to do it).

Each doc then has an "_readers" list.  It represents the list of authTokens
that have been given permission to read the document.

The filter query could then be something like "intersect(user.authTokens,
doc._readers).length > 0" the intersect command returns a list of
overlapping list members; the length of that list being the count of
members in common; and being greater than 0; means there was at least one

Other functions would be:
union: A list that is the combination of all lists with duplicates removed
filter: A list made from the first list, with all members of the other
lists removed
intersect: A list of only things in all lists
outersect: A list of only things that exist on one of the lists
member: A boolean where the length of the intersect > 0 (this would be
better used in the example above)

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