ruby-on-rails - `execute <<-SQL` 在 Rails 中是什么意思？

Question

当我在 http://guides.rubyonrails.org/active_record_migrations.html 中提到在线 active_record_migrations 时

在下面的片段中:

class ExampleMigration < ActiveRecord::Migration[5.0]
  def up
    create_table :distributors do |t|
      t.string :zipcode
    end

    # add a CHECK constraint
    execute <<-SQL
      ALTER TABLE distributors
        ADD CONSTRAINT zipchk
        CHECK (char_length(zipcode) = 5);
    SQL

    add_column :users, :home_page_url, :string
    rename_column :users, :email, :email_address
  end

  def down
    rename_column :users, :email_address, :email
    remove_column :users, :home_page_url

    execute <<-SQL
      ALTER TABLE distributors
        DROP CONSTRAINT zipchk
    SQL

    drop_table :distributors
  end
end

下面的语句是什么意思？

execute <<-SQL
  ALTER TABLE distributors
    ADD CONSTRAINT zipchk
    CHECK (char_length(zipcode) = 5);
SQL

使用 rails db:migrate 运行此迁移，我收到错误:

SQLite3::SQLException: near "CONSTRAINT": syntax error:           ALTER TABLE distributors
            ADD CONSTRAINT zipchk
              CHECK (char_length(zipcode) = 5) NO INHERIT;

引用3.9 Using reversible了解更多详情。

Answer 1

它叫做 heredoc与迁移、SQL 或其他任何具体内容无关:

If you are writing a large block of text you may use a “here document” or “heredoc”:

expected_result = <<HEREDOC

This would contain specially formatted text.

That might span many lines
HEREDOC

The heredoc starts on the line following << HEREDOC and ends with the next line that starts with HEREDOC. The result includes the ending newline.

You may use any identifier with a heredoc, but all-uppercase identifiers are typically used.

You may indent the ending identifier if you place a “-” after <<:

  expected_result = <<-INDENTED_HEREDOC
This would contain specially formatted text.

That might span many lines
  INDENTED_HEREDOC

Note that the while the closing identifier may be indented, the content is always treated as if it is flush left. If you indent the content those spaces will appear in the output.

自 ActiveRecord::ConnectionAdapters::DatabaseStatements#execute将一个字符串作为参数，您传递的是这个字符串，格式正确。