DEV: Adds a new converter for migrating from Discourse

It only contains a few user-related steps right now

migrations/lib/database/adapter/postgres.rb

@@ -0,0 +1,64 @@
+# frozen_string_literal: true
+
+require "pg"
+
+module Migrations::Database::Adapter
+  class Postgres
+    def initialize(settings)
+      @connection = PG::Connection.new(settings)
+      @connection.type_map_for_results = PG::BasicTypeMapForResults.new(@connection)
+      @connection.field_name_type = :symbol
+      configure_connection
+    end
+
+    def exec(sql)
+      @connection.exec(sql)
+    end
+
+    def query(sql, *params)
+      @connection.send_query_params(sql, params)
+      @connection.set_single_row_mode
+
+      Enumerator.new do |y|
+        while (result = @connection.get_result)
+          result.stream_each { |row| y.yield(row) }
+          result.clear
+        end
+      end
+    end
+
+    def query_first(sql)
+      @connection.exec(sql).first
+    end
+
+    def query_value(sql, column)
+      query_first(sql)[column]
+    end
+
+    def count(sql)
+      query_first(sql).values.first.to_i
+    end
+
+    def close
+      unless @connection&.finished?
+        @connection.finish
+        @connection = nil
+      end
+    end
+
+    def reset
+      @connection.reset
+      configure_connection
+    end
+
+    def escape_string(str)
+      @connection.escape_string(str)
+    end
+
+    private
+
+    def configure_connection
+      @connection.exec("SET client_min_messages TO WARNING")
+    end
+  end
+end