Support filtering on unexisting fields

[MarinPostma]

I realised that documents that don't have a value for a specific field are discarded. The question arises as how to handle them? Is an empty value the same thing as an unexisting field? I think not, but again this is debatable.

On a semantic level, the empty string is the only way we have to tell that we expect an empty value.

# this can't be a valid syntax
color = OR color = green

And the fact that we allow unquoted word a a value prevents us from adding a null keyword (which I'm against anyway).

Now we could have the following

# matches all the documents that do not have a field `color`
NOT color
# matches all the documents that have a field `color`
color

but that implies a consequent change in the syntax.

[vmitchell85]

+1 I would also like this feature.... I ended up moving from MeiliSearch because I couldn't do this.

[gmourier]

Hey 👋

This may interest you @vmitchell85 #407 (comment)! A prototype adding 2 new operators is available. It adds IN and EXISTS.

[gmourier]

Hey everyone 👋

I'm locking this discussion in favor of #603

Thanks!