Add instance search feature (#4925)

app/models/account.rb

@@ -104,6 +104,7 @@ class Account < ApplicationRecord
   scope :by_domain_accounts, -> { group(:domain).select(:domain, 'COUNT(*) AS accounts_count').order('accounts_count desc') }
   scope :matches_username, ->(value) { where(arel_table[:username].matches("#{value}%")) }
   scope :matches_display_name, ->(value) { where(arel_table[:display_name].matches("#{value}%")) }
+  scope :matches_domain, ->(value) { where(arel_table[:domain].matches("%#{value}%")) }
 
   delegate :email,
            :current_sign_in_ip,

app/models/instance_filter.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+class InstanceFilter
+  attr_reader :params
+
+  def initialize(params)
+    @params = params
+  end
+
+  def results
+    scope = Account.remote.by_domain_accounts
+    params.each do |key, value|
+      scope.merge!(scope_for(key, value)) if value.present?
+    end
+    scope
+  end
+
+  private
+
+  def scope_for(key, value)
+    case key.to_s
+    when 'domain_name'
+      Account.matches_domain(value)
+    else
+      raise "Unknown filter: #{key}"
+    end
+  end
+end