Problem with serializing/deserializing index to/from JSON

After making an index like so:

index = lunr(function(){
      this.ref('id');
      this.field('title', {boost: 10});
      this.field('text');
});

And calling index.toJSON() to create a storable representation, and then reloading the index from it:

newIndex = lunr.Index.load(index.toJSON());

The new index seems to be “broken” in some way. Attempting a search, for instance, gives the error TypeError: Cannot read property 'tf' of undefined

Maybe I’m making some simple error here, but it seems a little strange.

[olivernn]

As @brockfanning mentioned you must serialise the index by using JSON.stringify:

var idx = lunr(function(){
      this.ref('id')
      this.field('title', { boost: 10 })
      this.field('text')
})

idx.add(doc)

JSON.stringify(idx)

The reason JSON.stringify is important is that it will recursively call toJSON on every object that toJSON returns. In most cases this doesn’t matter (which is why diagnosing this issue is tricky!) however there are a couple of objects that act as stores for other objects, specifically lunr.Store. The return value from just calling toJSON on lunr.Store and the output of JSON.parse(JSON.stringify(store)) are different. This is because JSON.stringify is going through each item in the store and making sure toJSON is called, just calling lunr.Store.prototype.toJSON does not do this.

I think this is a case where the documentation could be improved, making it clearer how to serialise an index as well as marking the toJSON methods as private. I am currently working on a big overhaul of the documentation for lunr so hopefully other people will not run into the same problem.

[brockfanning]

Aha, I get the error now too.

Well, not sure why, but apparently stringifying and parsing it is the reason I wasn’t seeing that error in my own app:

index = lunr(function(){
      this.ref('id');
      this.field('title', {boost: 10});
      this.field('text');
});
index.add({id:1,title:'My Title',text:'My Text'});
indexStringified = JSON.stringify(index.toJSON());
indexParsed = JSON.parse(indexStringified);
newIndex = lunr.Index.load(indexParsed);
newIndex.search('text');