Add conversations API (#8832)

* Add conversations API

* Add web UI for conversations

* Add test for conversations API

* Add tests for ConversationAccount

* Improve web UI

* Rename ConversationAccount to AccountConversation

* Remove conversations on block and mute

* Change last_status_id to be a denormalization of status_ids

* Add optimistic locking

app/models/account_conversation.rb

@@ -0,0 +1,111 @@
+# frozen_string_literal: true
+# == Schema Information
+#
+# Table name: account_conversations
+#
+#  id                      :bigint(8)        not null, primary key
+#  account_id              :bigint(8)
+#  conversation_id         :bigint(8)
+#  participant_account_ids :bigint(8)        default([]), not null, is an Array
+#  status_ids              :bigint(8)        default([]), not null, is an Array
+#  last_status_id          :bigint(8)
+#  lock_version            :integer          default(0), not null
+#
+
+class AccountConversation < ApplicationRecord
+  after_commit :push_to_streaming_api
+
+  belongs_to :account
+  belongs_to :conversation
+  belongs_to :last_status, class_name: 'Status'
+
+  before_validation :set_last_status
+
+  def participant_account_ids=(arr)
+    self[:participant_account_ids] = arr.sort
+  end
+
+  def participant_accounts
+    if participant_account_ids.empty?
+      [account]
+    else
+      Account.where(id: participant_account_ids)
+    end
+  end
+
+  class << self
+    def paginate_by_id(limit, options = {})
+      if options[:min_id]
+        paginate_by_min_id(limit, options[:min_id]).reverse
+      else
+        paginate_by_max_id(limit, options[:max_id], options[:since_id])
+      end
+    end
+
+    def paginate_by_min_id(limit, min_id = nil)
+      query = order(arel_table[:last_status_id].asc).limit(limit)
+      query = query.where(arel_table[:last_status_id].gt(min_id)) if min_id.present?
+      query
+    end
+
+    def paginate_by_max_id(limit, max_id = nil, since_id = nil)
+      query = order(arel_table[:last_status_id].desc).limit(limit)
+      query = query.where(arel_table[:last_status_id].lt(max_id)) if max_id.present?
+      query = query.where(arel_table[:last_status_id].gt(since_id)) if since_id.present?
+      query
+    end
+
+    def add_status(recipient, status)
+      conversation = find_or_initialize_by(account: recipient, conversation_id: status.conversation_id, participant_account_ids: participants_from_status(recipient, status))
+      conversation.status_ids << status.id
+      conversation.save
+      conversation
+    rescue ActiveRecord::StaleObjectError
+      retry
+    end
+
+    def remove_status(recipient, status)
+      conversation = find_by(account: recipient, conversation_id: status.conversation_id, participant_account_ids: participants_from_status(recipient, status))
+
+      return if conversation.nil?
+
+      conversation.status_ids.delete(status.id)
+
+      if conversation.status_ids.empty?
+        conversation.destroy
+      else
+        conversation.save
+      end
+
+      conversation
+    rescue ActiveRecord::StaleObjectError
+      retry
+    end
+
+    private
+
+    def participants_from_status(recipient, status)
+      ((status.mentions.pluck(:account_id) + [status.account_id]).uniq - [recipient.id]).sort
+    end
+  end
+
+  private
+
+  def set_last_status
+    self.status_ids     = status_ids.sort
+    self.last_status_id = status_ids.last
+  end
+
+  def push_to_streaming_api
+    return if destroyed? || !subscribed_to_timeline?
+    PushConversationWorker.perform_async(id)
+  end
+
+  def subscribed_to_timeline?
+    Redis.current.exists("subscribed:#{streaming_channel}")
+  end
+
+  def streaming_channel
+    "timeline:direct:#{account_id}"
+  end
+end