Search Implementation

Tachi's search implementation uses MongoDB's $text index. This breaks a query into words and compares each of them to the provided text fields.

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__textScore

For our code, we mutate the documents we want to return with a special field: __textScore.

This field declares how 'close' the provided query was to the $text fields in this document.

This is sometimes exposed in the API for sorting reasons.

Bug

MongoDB's $text matching algorithm isn't great for fuzzy matches - It doesn't like song titles like 'A', as it thinks 'a' is an article, and doesn't match it properly as a word.

Info

Why not regex for fuzzy matches?

Regex has performance issues on larger datasets and we want to avoid it. Most regexes cannot use indexes, and therefore invoke a COLLSCAN, which we want to avoid.

User Searching

Searching users, on the other hand, has to use regex-based searching.

The $text method attempts to break things up based on their words, but that doesn't help with usernames, as they are all too frequently XxX_One_Long_Str1ng_xXx.

Instead, we use a case insensitive regex - similar to SQL's LIKE.

This means we do not have a __textScore property for this search to sort on. Instead, we just constrict returns to around 15, and have the user whittle their search down better.