MatchEngine is an in-memory matching/filtering engine with MongoDB-like query syntax.

Introduction

The query language consists of nested Elixir "keyword list". Each component of the query consists of a key part and a value part. The key part is either a logic operator (and/or/not), or a reference to a field, the value part is either a plain value, or a value operator.

When a query is run against a document, where each term is scored individually and then summed. (This implies "or"). Some example queries:

[title: "hoi"]
[title: [_eq: "hoi"]]
[_and: [name: "John", age: 36]]
[_or: [name: "John", age: 36]]
[_not: [title: "foo"]]

Two ways of saying "Score all documents in which the title equals "hoi"":

[title: "hoi"]
[title: [_eq: "hoi"]]

Combining various matchers with logic operators:

[_and: [name: "John", age: 36]]
[_or: [name: "John", age: 36]]
[_not: [title: "foo"]]

Performing matches in nested objects is also possible; the query simply follows the shape of the data.

Given a document consisting of a nested structure, %{"user" => %{"name" => "John"}}:

"User name equals John":

[user: [name: "John"]]

"User name does not equal John":

[_not: [user: [name: "John"]]]

Note that this is a different approach for nesting fields than MongoDB, which uses dot notation for field nesting.

Query execution

The queries can be run by calling MatchEngine.score_all/2 or MatchEngine.filter_all/2.

Queries are first preprocessed, and then executed on a list of search "documents". A "document" is just a normal Elixir map, with string keys.

The preprocessing phase compiles any regexes, checks whether all operators exist, and de-nests nested field structures.

The query phase runs the preprocessed query for each document in the list, by calculating the score for the given document, given the query. When using filter_all/2, documents with a zero score are removed from the input list. When using score_all, the list is sorted on score, descending, and this score, including any additional metadata, is returned in a "_match" map inside the document.

Value operators

Value operators work on an individual field. Various operators can be used to calculate a score for a given field.

_eq

Scores on the equality of the argument.

[title: "hello"]
[title: [_eq: "hello"]]

_ne

Scores on the inequality of the argument. ("Not equals")

[title: [_ne: "hello"]]

_has

Scores when the document's value is a member of the given list.

[role: [_in: ["developer", "freelancer"]]]

_hasnt

Scores when the document's value contains a member of the given list or contains the given word or words

[tag: [_has: ["production"]]]
[title: [_has: "The"]]
[title: [_has: ["The", "title"]]]

_in

Scores when the document's value is a member of the given list.

[role: [_in: ["developer", "freelancer"]]]

_nin

Scores when the document's value is not a member of the given list.

[role: [_nin: ["recruiter"]]]

_lt, _gt, _lte, _gte

Scores on using the comparison operators <, >, <= and >=.

[age: [_gt: 18]]

_sim

Normalized string similarity. The max of the Normalised Levenshtein distance and Jaro distance.

_regex

Match a regular expression. The input is a string, which gets compiled into a regex. This operator scores on the length of match divided by the total string length. It is possible to add named captures to the regex, which then get added to the _match metadata map, as seen in the following exapmle:

# regex matches entire string, 100% score
assert %{"score" => 1} == score([title: [_regex: "foo"]], %{"title" => "foo"})
# regex matches with a capture called 'name'. It is boosted by weight.
assert %{"score" => 1.6, "name" => "food"} == score([title: [_regex: "(?P<name>foo[dl])", w: 4]], %{"title" => "foodtrucks"})

The regex match can also be inversed, where the document value is treated as the regular expression, and the query input is treated as the string to be matched. (No captures are supported in this case).

assert %{"score" => 0.5} == score([title: [_regex: "foobar", inverse: true]], %{"title" => "foo"})

_geo

Calculate document score based on its geographical distance to a given point. The geo distance (both in the operator and in the document) can be given as:

• A regular list, e.g. [4.56, 52.33]
• A keyword list, e.g. [lat: 52.33, lon: 4.56]
• A map with atom keys, e.g. %{lat: 52.33, lon: 4.56}
• A map with string keys, e.g. %{"lat" => 52.33, "lon" => 4.56}

The calculated distance is returned in meters, as part of the _match map.

An extra argument, max_distance can be given to the operator which specifies the maximum cutoff point. It defaults to 100km. (100_000). Distance is scored logarithmically with respect to the maximum distance.

doc = %{"location" => %{"lat" => 52.340500999999996, "lon" => 4.8832816}}
q = [location: [_geo: [lat: 52.340500999999996, lon: 4.8832816]]]
assert %{"score" => 1, "distance" => 0.0} == score(q, doc)

When radius is given as an option, all geo points that are within the radius will score a 1 and the max_distance scoring will be in effect for distances larger than the radius.

_geo_poly

Calculate document score based on its containment inside a given geographical polygon.

Accepts a list of geographical coordinates, each in the same format as _geo.

Like _geo, the max_distance option can be given to the operator which specifies the maximum cutoff point. It defaults to 100km. (100_000). Distance is scored logarithmically with respect to the maximum distance.

When the point is inside the polygon, the score is always 1. Only when the point is outside the polygon, the geographical distance from the document point to the closest point on the edge of the polygon is calculated and scored based on the max_distance setting.

_time

Score by an UTC timestamp, relative to the given time.

t1 = "2018-02-19T15:29:53.672235Z"
t2 = "2018-02-19T15:09:53.672235Z"
assert %{"score" => s} = score([inserted_at: [_time: t1]], %{"inserted_at" => t2})

This way, documents can be returned in order of recency.

Logic operators

_and

Combine matchers, multiplying the score. When one of the matchers returns 0, the total score is 0 as well.

[_and: [name: "John", age: 36]]

_or

Combine matchers, adding the scores.

[_or: [name: "John", id: 12]]

_not

Reverse the score of the nested matchers. (when score > 0, return 0, otherwise, return 1.

[_not: [title: "foo"]]

Matcher weights

w: 10 can be added to a matcher term to boost its score by the given weight.

[title: [_eq: "Pete", w: 5], summary: [_sim: "hello", w: 2]]

b: true can be added to force a score of 1 when the score is > 0.

[title: [_sim: "hello", b: true]]

Map syntax for queries

Instead of keyword lists, queries can also be specified as maps. In this case, the keys of the map need to be strings. Query maps are meant to be used from user-generated input, and can be easily created from JSON files.

[_not: [title: "foo"]]
# can also be written as:
%{"_not" => %{"title" => "foo"}}

[title: [_eq: "Pete", w: 5], summary: [_sim: "hello", w: 2]]
# can also be written as:
%{"title" => %{"_eq" => "Pete", "w" => 5}, "summary" => %{"_sim" => "hello", "w" => 2}}