fix(poucdb-find-mergeAndedSelectors): improve handling of merging of selectors

[TamimEhsan]

Setup

Let's consider a case where the massageSelector fails to render some branches

[
    {_id: '1', a: 1, b: 2},
    {_id: '2', a: 1, b: 4},
    {_id: '3', a: 3, b: 2},
    {_id: '4', a: 3, b: 4},
    {_id: '5', a: 5, b: 6}
 ]

with query

query = {
    fields: ["_id"],
    selector: {
      $and: [
        {
            $or: [
                {a: 1},
                {b: {$gte:2}}
            ]
        },
        {
            $or: [
                {a: 3},
                {b: {$gte:4}}
            ]
        }
    ]
    }
  };

We expect that the doc with _id : 5 to be returned. But the actual return doesn't include that.

Possible problem

the two selector {b: {$gte:2}} and {b: {$gte:4}} gets merged into one {b: {$gte:4}}. However, due to the logic implemented at the below segment, we lose this merged selector.
https://github.com/pouchdb/pouchdb/blob/f2c665a2a885437b9cea80dda62c02a93a137c1e/packages/node_modules/pouchdb-selector-core/src/utils.js#L96-L101

We can come up with more of similar cases where we lose the selector. So, removing it might be a good approach. However, then we actually fail to cover the case it was actually meant to handle ie {b: {$eq:2}} and {b: {$eq:4}}. Which should not be merged into {b: {$eq:4}} due to the current implementation of mergeEq function
https://github.com/pouchdb/pouchdb/blob/f2c665a2a885437b9cea80dda62c02a93a137c1e/packages/node_modules/pouchdb-selector-core/src/utils.js#L212-L221

Proposed Solution

I propose a simple fix to this. At first we can remove code segment if (Object.keys(merged).length <= longest) { return; } . Then for conflicting $eq conditions, we add a $ne to the condition, thus causing the selector to be false by default. Although this is a bit hacky, but it will make the logic a bit easier. Later, we can filter the selector based on this contradiction instead of length.

function mergeEq(value, fieldMatchers) {
  // these all have less specificity than the $eq
  // TODO: check for user errors here
  delete fieldMatchers.$gt;
  delete fieldMatchers.$gte;
  delete fieldMatchers.$lt;
  delete fieldMatchers.$lte;
  delete fieldMatchers.$ne;
+  if ( '$eq' in fieldMatchers && fieldMatchers.$eq !== value ) {
+   mergeNe(value, fieldMatchers);
+ }
  fieldMatchers.$eq = value;
}

Alternative Solution

Alternatively, we can also introduce a new field like $falacy or something, which will be set to true when such contradictions arise. Then like before we can filter the selector based on that.

Testing

I have tested the change against some edge cases. So, far it seems to be working. And it passes all the tests too for TYPE=find PLUGINS=pouchdb-find COUCH_HOST='http://admin:password@localhost:5984' npm test

Note 1

Although $eq has more specificity than the others, but deleting the other operators might not be the best choice here. Having an operator like $gte : 10 and merging it with $eq : 5 should keep both, as the correct merged selector is an always false selector. But deleting $gte will cause it to change its behavior.