Add cold-start follow recommendations (#15945)

app/models/follow_recommendation.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+# == Schema Information
+#
+# Table name: follow_recommendations
+#
+#  account_id :bigint(8)        primary key
+#  rank       :decimal(, )
+#  reason     :text             is an Array
+#
+
+class FollowRecommendation < ApplicationRecord
+  self.primary_key = :account_id
+
+  belongs_to :account_summary, foreign_key: :account_id
+  belongs_to :account, foreign_key: :account_id
+
+  scope :safe, -> { joins(:account_summary).merge(AccountSummary.safe) }
+  scope :localized, ->(locale) { joins(:account_summary).merge(AccountSummary.localized(locale)) }
+  scope :filtered, -> { joins(:account_summary).merge(AccountSummary.filtered) }
+
+  def readonly?
+    true
+  end
+
+  def self.get(account, limit, exclude_account_ids = [])
+    account_ids = Redis.current.zrevrange("follow_recommendations:#{account.user_locale}", 0, -1).map(&:to_i) - exclude_account_ids - [account.id]
+
+    return [] if account_ids.empty? || limit < 1
+
+    accounts = Account.followable_by(account)
+                      .not_excluded_by_account(account)
+                      .not_domain_blocked_by_account(account)
+                      .where(id: account_ids)
+                      .limit(limit)
+                      .index_by(&:id)
+
+    account_ids.map { |id| accounts[id] }.compact
+  end
+end