Implements the EXISTS filter

[irevoire]

After the following discussion and a meeting with @gmourier and @loiclec; https://github.com/meilisearch/product/issues/22

The first version of the filter should be implemented with the following syntax;

The keyword is postfixed
       vvvvv
 price EXISTS
^     ^
A NOT can be prefixed or infixed
vvv       vvv
NOT price NOT EXISTS

With the following set of documents;

{ "id": 1, "product": "T-shirt", "price": 20, "color": "yellow" }
{ "id": 2, "product": "T-shirt", "color": "red" }

• The filter price EXISTS will select the first document.
• The filter price NOT EXISTS or NOT price EXISTS will select only the second document.

If a field contains an empty array or a null value, it's considered as existing:

{ "id": 1, "product": "T-shirt", "price": 20, "color": "yellow" }
{ "id": 2, "product": "T-shirt", "price": [], "color": "red" }
{ "id": 3, "product": "T-shirt", "price": null, "color": "red" }
{ "id": 4, "product": "T-shirt", "color": "red" }

Here price EXISTS matches documents 1, 2 and 3.

---

This will ease the handling of incomplete documents. For example, if you want to return all T-shirt that cost less than 20€ or that doesn't have a price specified, you will be able to write product = "T-shirt" AND (price < 20 OR price NOT EXISTS).
That was not possible previously.

[curquiza]

For people following this issue, we have already published a docker tag to test the feature

docker run -it --rm \
    -p 7700:7700 \
    getmeili/meilisearch:v0.29.0-filter.beta.0

Or you can compile the source code on the filter/field-in branch on this repo

All the information about the new addition is detailed here

Any feedback is more than welcome!! ❤️

[mech]

I am curious, if my field got value null, does this EXISTS work? I am trying to do a filter UI with the operator option named "Is empty" and "Is not empty" sort of like Notion filter. But it seems like this is not possible.

Will there be any support in the works or we need to put a value like field = '__SPECIAL_NULL__' fake value to circumvent it?

[irevoire]

Hey @mech,
Currently, the exists filter matches the documents where the field exists.
Existing means the field in the JSON being equal to null doesn't change anything about its existence.

If a field contains an empty array or a null value, it's considered as existing.

What you're asking for would be another feature we've already thought about. It's not planned currently, but I'm going to open a discussion on our product repository, and it would be nice if you could answer me over there so we don't lose any information!
meilisearch/product#539