rake db:migrate 总是输出 SELECT "schema_migrations".* FROM "schema_migrations"

Question

每次我运行 rake db:migrate 我都会得到以下输出：

  ActiveRecord::SchemaMigration Load (0.9ms)  SELECT "schema_migrations".* FROM "schema_migrations"
  ActiveRecord::SchemaMigration Load (0.2ms)  SELECT "schema_migrations".* FROM "schema_migrations"

这在安装rspec-rails gem 后开始。

当我尝试运行测试时，我得到以下输出：

  ActiveRecord::SchemaMigration Load (0.4ms)  SELECT "schema_migrations".* FROM "schema_migrations"
   (0.2ms)  BEGIN
Processing by WelcomeController#index as HTML
  Rendered welcome/index.html.erb within layouts/application (0.3ms)
Completed 200 OK in 17ms (Views: 8.3ms | ActiveRecord: 3.1ms)
   (0.2ms)  ROLLBACK
.   (0.1ms)  BEGIN
Processing by WelcomeController#index as HTML
Completed 200 OK in 1ms (Views: 1.0ms | ActiveRecord: 0.0ms)
   (0.1ms)  ROLLBACK
.

Finished in 0.03 seconds (files took 1.5 seconds to load)
2 examples, 0 failures

我创建了 test_database 并运行了迁移。任何想法是什么导致了这个错误？

Answer 1

Rspec-rails 生成了这个：它是在运行任何测试之前自动运行任何挂起的迁移的检查。它在您的/spec/rails_helpers.rb 文件中并说：

# Checks for pending migrations before tests are run.
# If you are not using ActiveRecord, you can remove this line.
ActiveRecord::Migration.maintain_test_schema!

将那一行保留在其中是一个非常好的主意，这样您就不会在数据库未处于良好/完整/预期状态时无意中运行测试（如果您忘记了迁移，可能会发生这种情况运行）。

Answer 2

tl;dr 这不是错误

schema_migrations 是一个表，rails 存储它为当前环境的数据库运行的所有迁移的版本号。

它会在迁移数据库时始终检查这一点（在规范之前它也会这样做），以查看是否有任何尚未为此数据库运行的未决迁移。