Fix tag search order and not to use tsvector (#3611)

* Sort results by the name
* Switch search method to simple `LIKE` matching instead of tsvector/tsquery

Previously we used scores from ts_rank_cd() to sort results, but it didn't work
because the function returns same score for all results. It's not for calculate
similarity of single words. Sometimes this bug even push out exact matching tag
from results.

Additionally, PostgreSQL supports prefix searching with standard btree index.
Using it offers simpler code, but also less index size and some speed.

app/models/tag.rb

@@ -21,22 +21,9 @@ class Tag < ApplicationRecord
   end
 
   class << self
-    def search_for(terms, limit = 5)
-      terms      = Arel.sql(connection.quote(terms.gsub(/['?\\:]/, ' ')))
-      textsearch = 'to_tsvector(\'simple\', tags.name)'
-      query      = 'to_tsquery(\'simple\', \'\'\' \' || ' + terms + ' || \' \'\'\' || \':*\')'
-
-      sql = <<-SQL.squish
-        SELECT
-          tags.*,
-          ts_rank_cd(#{textsearch}, #{query}) AS rank
-        FROM tags
-        WHERE #{query} @@ #{textsearch}
-        ORDER BY rank DESC
-        LIMIT ?
-      SQL
-
-      Tag.find_by_sql([sql, limit])
+    def search_for(term, limit = 5)
+      pattern = sanitize_sql_like(term) + '%'
+      Tag.where('name like ?', pattern).order(:name).limit(limit)
     end
   end
 end

db/migrate/20170606113804_change_tag_search_index_to_btree.rb

+class ChangeTagSearchIndexToBtree < ActiveRecord::Migration[5.1]
+
+  def up
+    remove_index :tags, name: :hashtag_search_index
+    execute 'CREATE INDEX hashtag_search_index ON tags (name text_pattern_ops);'
+  end
+
+  def down
+    remove_index :tags, name: :hashtag_search_index
+    execute 'CREATE INDEX hashtag_search_index ON tags USING gin(to_tsvector(\'simple\', tags.name));'
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170604144747) do
+ActiveRecord::Schema.define(version: 20170606113804) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -320,7 +320,7 @@ ActiveRecord::Schema.define(version: 20170604144747) do
     t.string "name", default: "", null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
-    t.index "to_tsvector('simple'::regconfig, (name)::text)", name: "hashtag_search_index", using: :gin
+    t.index "name text_pattern_ops", name: "hashtag_search_index"
     t.index ["name"], name: "index_tags_on_name", unique: true
   end
 

spec/models/tag_spec.rb

@@ -22,5 +22,14 @@ RSpec.describe Tag, type: :model do
 
       expect(results).to eq [tag]
     end
+
+    it 'finds the exact matching tag as the first item' do
+      similar_tag = Fabricate(:tag, name: "matchlater")
+      tag = Fabricate(:tag, name: "match")
+
+      results = Tag.search_for("match")
+
+      expect(results).to eq [tag, similar_tag]
+    end
   end
 end