Calculate relevant result on full text search in mongodb - mongodb

Question

I am trying to get the more relevant results from mongo, let say that i have this collections
{ "text" : "mitsubishi lancer 2011"}
{ "text" : "mitsubishi lancer 2011"}
{ "text" : "mitsubishi lancer 2011 in good conditions"}
{ "text" : "lancer 2011"}
{ "text" : "mitsubishi lancer 2014"}
{ "text" : "lancer 2016"}
and make this query
db.post.find({$text: {$search: "mitsubishi lancer 2011"}}, {score: {$meta: "textScore"}}).sort({score:{$meta:"textScore"}})
i get this result
{ "text" : "mitsubishi lancer 2011", "score" : 2 }
{ "text" : "mitsubishi lancer 2011", "score" : 2 }
{ "text" : "mitsubishi lancer 2011 in good conditions", "score" : 1.7999999999999998 }
{ "text" : "lancer 2011", "score" : 1.5 }
{ "text" : "mitsubishi lancer 2014", "score" : 1.3333333333333333 }
{ "text" : "lancer 2016", "score" : 0.75 }
How do i know that the first two have all the text that i search?
who the score is calculated?

Answer 1

The scoring algorithm is internal to MongoDB and should probably be expected to change over time so the precise values shouldn't matter. You can attempt to understand what's going on by looking at the sources if you want (although I wouldn't recommend that).
The final score depends on the number of occurrences of your searched terms (or rather their word stems), the distances between the matches, the match quality (full match vs. partial), language settings and weights which you can configure. That's all pretty hefty stuff that cannot easily be documented. There is, however, a blog post that explains some aspects quite nicely: https://blog.codecentric.de/en/2013/01/text-search-mongodb-stemming/
Also, things get a bit clearer once you try out various queries using different combinations of search terms and indexed data.
Lastly, if you want to find out if there is a perfect match, the only way I can think of to make this work is something like this:
db.getCollection('test').aggregate(
{
// do the normal filtering query
$match: {
$text: {
$search: "mitsubishi lancer 2011"
}
}
}, {
// select what's relevant in the output and add an indicator "perfectmatch"
$project: {
"text": 1,
"score": {
$meta: "textScore"
},
"perfectmatch": {
$cond: [
{ $eq: [ "$text", "mitsubishi lancer 2011" ] }, // this would check for a perfect match using the exact full string, for individual token matching you would need to do tokenize your query and do a series of other checks here.
true,
false
]
}
}
}, {
// if you want to have the results sorted by "best match first"
$sort: {
"score": -1
}
})