小编Leo*_*eoG的帖子

使用gradle 4.3在Windows 7上构建一个react-native应用程序,我得到以下错误,不确定是什么问题,任何帮助将是apreciated.

C:\Users\username\AwesomeProject>react-native run-android
Scanning folders for symlinks in C:\Users\username\AwesomeProject\node_modules (62ms)
Starting JS server...
Building and installing the app on the device (cd android && gradlew.bat installDebug)...
Starting a Gradle Daemon (subsequent builds will be faster)
Download https://jcenter.bintray.com/com/facebook/react/react-native/maven-metadata.xml

FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring project ':app'.
> Failed to notify project evaluation listener.
   > com.android.build.gradle.tasks.factory.AndroidJavaCompile.setDependencyCacheDir(Ljava/io/File;)V

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug …

我的代码,

class User(db.Model, UserMixin):
  uid = db.Column(db.Integer, primary_key=True)
  username = db.Column(db.String(250), unique=True)
  password = db.Column(db.String(250))
  name = db.Column(db.String(250))
  created_on = db.Column(db.TIMESTAMP, default=False)
  modified_on = db.Column(db.TIMESTAMP, onupdate=datetime.datetime.now)

我想创建一个字段时间戳,它将具有默认值时间戳,onupdate将再次更改为当前时间戳.但是当我运行迁移时,这不会在MySQL中使用默认值或更新值创建字段,我做错了什么？

我知道我需要在SQLAlchemy中使用MetaData对象,但我不确定如何在类中使用它,

db = SQLAlchemy(app)
meta =db.Metadata()
class orders(db.model):
  pass

如何将元对象传递给类,以便它自动生成表模式？