abc.map do |i|
  i['id']  
end

没有区别

在哪里？可能要用 https://ruby-doc.org/stdlib-2.5.0/libdoc/uri/rdoc/URI.html

don't worry, it cannot update your system, just install some requirements

每个 if(不是 elsif) 都要有自己的 end. 下面也是可以的

def tt(n)
  if n == 1
    p 1 
  else if n == 2
      p  2
    elsif n== 3
      p 3 
    end
  end 
end

又及：

def tt(n)
  if n == 1
    p 1 
  else 
    if n == 2
      p  2
    elsif n== 3
      p 3 
    end
  end 
end

if..[elsif]..[else]...end 是一个 statement

https://www.railstutorial.org/

Documentation says "You may need to install a package such as libmysqlclient-dev or mysql-devel"

Try to update, it should works well on 5.2+

and what version of Rails? PostgreSQL 10 supports since 5.2.0 beta

Haven't you updated PostgreSQL from 9 to 10? I had same error, because PostgreSQL 10 has another format of sequences.

请给我们看在 Gemfile declarative 的行

附：确定您在 Gemfile 有‘ruby x.x.x.' 行，x.x.x 是 ruby 的版本

http://www.exercism.io/