[#1570] Experimental feature to improve user search using Levenshtein distance calculation.

Improves short queries, especially containing typos.

lib/pleroma/user.ex

@@ -92,6 +92,7 @@ defmodule Pleroma.User do
     field(:local, :boolean, default: true)
     field(:follower_address, :string)
     field(:following_address, :string)
+    field(:levenshtein_distance, :integer, virtual: true)
     field(:search_rank, :float, virtual: true)
     field(:search_type, :integer, virtual: true)
     field(:tags, {:array, :string}, default: [])

lib/pleroma/user/search.ex

@@ -8,6 +8,8 @@ defmodule Pleroma.User.Search do
   import Ecto.Query
 
   @limit 20
+  @levenshtein_max_query_length 5
+  @search_rank_threshold 0
 
   def search(query_string, opts \\ []) do
     resolve = Keyword.get(opts, :resolve, false)
@@ -31,7 +33,10 @@ defmodule Pleroma.User.Search do
 
   defp format_query(query_string) do
     # Strip the beginning @ off if there is a query
-    query_string = String.trim_leading(query_string, "@")
+    query_string =
+      query_string
+      |> String.trim()
+      |> String.trim_leading("@")
 
     with [name, domain] <- String.split(query_string, "@") do
       encoded_domain =
@@ -47,15 +52,33 @@ defmodule Pleroma.User.Search do
     end
   end
 
+  defp levenshtein_applicable?(query_string) do
+    String.length(query_string) <= @levenshtein_max_query_length
+  end
+
   defp search_query(query_string, for_user, following) do
-    for_user
-    |> base_query(following)
-    |> filter_blocked_user(for_user)
-    |> filter_invisible_users()
-    |> filter_blocked_domains(for_user)
-    |> fts_search(query_string)
-    |> trigram_rank(query_string)
+    query =
+      for_user
+      |> base_query(following)
+      |> filter_blocked_user(for_user)
+      |> filter_invisible_users()
+      |> filter_blocked_domains(for_user)
+
+    query =
+      if levenshtein_applicable?(query_string) do
+        query
+        |> levenshtein_distance(query_string)
+        |> fts_levenshtein_search(query_string)
+        |> trigram_levenshtein_rank(query_string)
+      else
+        query
+        |> fts_search(query_string)
+        |> trigram_rank(query_string)
+      end
+
+    query
     |> boost_search_rank(for_user)
+    |> filter_by_search_rank()
     |> subquery()
     |> order_by(desc: :search_rank)
     |> maybe_restrict_local(for_user)
@@ -78,6 +101,25 @@ defmodule Pleroma.User.Search do
     )
   end
 
+  defp fts_levenshtein_search(query, query_string) do
+    tsquery = to_tsquery(query_string)
+
+    from(
+      u in subquery(query),
+      where:
+        fragment(
+          """
+          ? <= 2 OR
+            (to_tsvector('simple', ?) || to_tsvector('simple', ?)) @@ to_tsquery('simple', ?)
+          """,
+          u.levenshtein_distance,
+          u.name,
+          u.nickname,
+          ^tsquery
+        )
+    )
+  end
+
   defp to_tsquery(query_string) do
     String.trim_trailing(query_string, "@" <> local_domain())
     |> String.replace(~r/[!-\/|@|[-`|{-~|:-?]+/, " ")
@@ -87,6 +129,45 @@ defmodule Pleroma.User.Search do
     |> Enum.join(" | ")
   end
 
+  # Trigram-based rank with bonus for close Levenshtein distance b/w query and nickname
+  defp trigram_levenshtein_rank(query, query_string) do
+    from(
+      u in subquery(query),
+      select_merge: %{
+        search_rank:
+          fragment(
+            "similarity(?, trim(? || ' ' || coalesce(?, ''))) + \
+                            (CASE WHEN ? = 0 THEN 1.0 \
+                              WHEN ? = 1 AND length(?) > 1 THEN 0.5
+                              WHEN ? = 2 AND length(?) > 3 THEN 0.2
+                              ELSE 0 END)",
+            ^query_string,
+            u.nickname,
+            u.name,
+            u.levenshtein_distance,
+            u.levenshtein_distance,
+            ^query_string,
+            u.levenshtein_distance,
+            ^query_string
+          )
+      }
+    )
+  end
+
+  defp levenshtein_distance(query, query_string) do
+    from(
+      u in query,
+      select_merge: %{
+        levenshtein_distance:
+          fragment(
+            "levenshtein(?, regexp_replace(?, '@.+', ''))",
+            ^query_string,
+            u.nickname
+          )
+      }
+    )
+  end
+
   defp trigram_rank(query, query_string) do
     from(
       u in query,
@@ -185,4 +266,8 @@ defmodule Pleroma.User.Search do
   end
 
   defp boost_search_rank(query, _for_user), do: query
+
+  defp filter_by_search_rank(query) do
+    from(u in subquery(query), where: u.search_rank > @search_rank_threshold)
+  end
 end

priv/repo/migrations/20200604150903_add_fuzzystrmatch_postgres_extension.exs

@@ -0,0 +1,23 @@
+defmodule Pleroma.Repo.Migrations.AddFuzzystrmatchPostgresExtension do
+  use Ecto.Migration
+
+  require Logger
+
+  def up do
+    Logger.warn("ATTENTION ATTENTION ATTENTION\n")
+
+    Logger.warn(
+      "This will try to create the pg_trgm extension on your database. If your database user does NOT have the necessary rights, you will have to do it manually and re-run the migrations.\nYou can probably do this by running the following:\n"
+    )
+
+    Logger.warn(
+      "sudo -u postgres psql pleroma_dev -c \"create extension if not exists fuzzystrmatch\"\n"
+    )
+
+    execute("create extension if not exists fuzzystrmatch")
+  end
+
+  def down do
+    execute("drop extension if exists fuzzystrmatch")
+  end
+end

test/tasks/user_test.exs

@@ -436,11 +436,14 @@ defmodule Mix.Tasks.Pleroma.UserTest do
 
       {:ok, user} = User.follow(user, kawen)
 
-      assert [moon.id, kawen.id] == User.Search.search("moon") |> Enum.map(& &1.id)
+      # One "typo" in nickname makes `moot` score better than `kawen` despite of name match
+      assert [moon.id, moot.id, kawen.id] == User.Search.search("moon") |> Enum.map(& &1.id)
+
       res = User.search("moo") |> Enum.map(& &1.id)
       assert moon.id in res
       assert moot.id in res
       assert kawen.id in res
+
       assert [moon.id, kawen.id] == User.Search.search("moon fediverse") |> Enum.map(& &1.id)
 
       assert [kawen.id, moon.id] ==

test/user_search_test.exs

@@ -37,6 +37,10 @@ defmodule Pleroma.UserSearchTest do
       assert length(User.search("john", limit: 3, offset: 3)) == 2
     end
 
+    defp clear_virtual_fields(user) do
+      Map.merge(user, %{search_rank: nil, search_type: nil, levenshtein_distance: nil})
+    end
+
     test "finds a user by full or partial nickname" do
       user = insert(:user, %{nickname: "john"})
 
@@ -44,8 +48,7 @@ defmodule Pleroma.UserSearchTest do
         assert user ==
                  User.search(query)
                  |> List.first()
-                 |> Map.put(:search_rank, nil)
-                 |> Map.put(:search_type, nil)
+                 |> clear_virtual_fields()
       end)
     end
 
@@ -56,8 +59,8 @@ defmodule Pleroma.UserSearchTest do
         assert user ==
                  User.search(query)
                  |> List.first()
-                 |> Map.put(:search_rank, nil)
-                 |> Map.put(:search_type, nil)
+                 |> Map.merge(%{search_rank: nil, search_type: nil, levenshtein_distance: nil})
+                 |> clear_virtual_fields()
       end)
     end
 
@@ -68,6 +71,23 @@ defmodule Pleroma.UserSearchTest do
       assert [u2.id, u1.id] == Enum.map(User.search("bar word"), & &1.id)
     end
 
+    test "considers Levenshtein distance between query and nickname for short queries" do
+      clear_config([:instance, :limit_to_local_content], false)
+
+      user = insert(:user, %{nickname: "hj@shigusegubu.club"})
+      insert(:user, %{nickname: "xyz@sample.com"})
+      insert(:user, %{nickname: "zyx@hj.com"})
+
+      # Note: "h.j." and "hhhj" are matched since 4+ char queries allow for 2 typos
+      for query <- ["hj", "hhj", "h j", "lj", "hi", "jj", "h.j.", "hhhj"] do
+        assert [user.id] == Enum.map(User.search(query), & &1.id)
+      end
+
+      for query <- ["ajay", "gg"] do
+        assert [] == User.search(query)
+      end
+    end
+
     test "finds users, boosting ranks of friends and followers" do
       u1 = insert(:user)
       u2 = insert(:user, %{name: "Doe"})