Python Sqlalchemy 如果表不存在
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/33053241/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Sqlalchemy if table does not exist
提问by jake wong
I wrote a module which is to create an empty database file
我写了一个模块来创建一个空的数据库文件
def create_database():
engine = create_engine("sqlite:///myexample.db", echo=True)
metadata = MetaData(engine)
metadata.create_all()
But in another function, I want to open myexample.dbdatabase, and create tables to it if it doesn't already have that table.
但是在另一个函数中，我想打开myexample.db数据库，如果它还没有该表，则为其创建表。
EG of the first, subsequent table I would create would be:
我将创建的第一个后续表的 EG 是：
Table(Variable_TableName, metadata,
Column('Id', Integer, primary_key=True, nullable=False),
Column('Date', Date),
Column('Volume', Float))
(Since it is initially an empty database, it will have no tables in it, but subsequently, I can add more tables to it. Thats what i'm trying to say.)
（由于它最初是一个空数据库，因此其中没有表，但随后，我可以向其中添加更多表。这就是我想说的。）
Any suggestions?
有什么建议？
采纳答案by jake wong
I've managed to figure out what I intended to do. I used engine.dialect.has_table(engine, Variable_tableName)to check if the database has the table inside. IFit doesn't, then it will proceed to create a table in the database.
我已经设法弄清楚我打算做什么。我曾经engine.dialect.has_table(engine, Variable_tableName)检查过数据库里面是否有表。如果没有，那么它将继续在数据库中创建一个表。
Sample code:
示例代码：
engine = create_engine("sqlite:///myexample.db") # Access the DB Engine
if not engine.dialect.has_table(engine, Variable_tableName): # If table don't exist, Create.
metadata = MetaData(engine)
# Create a table with the appropriate Columns
Table(Variable_tableName, metadata,
Column('Id', Integer, primary_key=True, nullable=False),
Column('Date', Date), Column('Country', String),
Column('Brand', String), Column('Price', Float),
# Implement the creation
metadata.create_all()
This seems to be giving me what i'm looking for.
这似乎给了我我正在寻找的东西。
回答by Belle05
Note that in 'Base.metadata' documentationit states about create_all:
请注意，在“Base.metadata”文档中，它说明了 create_all：
Conditional by default, will not attempt to recreate tables already present in the target database.
默认情况下有条件，不会尝试重新创建目标数据库中已经存在的表。
And if you can see that create_all takes these arguments: create_all(self, bind=None, tables=None, checkfirst=True), and according to documentation:
如果您可以看到 create_all 接受以下参数：create_all(self, bind=None, tables=None, checkfirst=True)，并且根据文档：
Defaults to True, don't issue CREATEs for tables already present in the target database.
默认为 True，不要为目标数据库中已经存在的表发出 CREATE。
So if I understand your question correctly, you can just skip the condition.
所以如果我正确理解你的问题，你可以跳过条件。
回答by Ricky Levi
For those who define the table first in some models.tablefile, among other tables.
This is a code snippet for finding the class that represents the table we want to create ( so later we can use the same code to just query it )
对于那些首先在某个models.table文件中定义表的人，以及其他表。这是一个代码片段，用于查找代表我们要创建的表的类（以便稍后我们可以使用相同的代码来查询它）
But together with the ifwritten above, I still run the code with checkfirst=True
但是加上if上面写的，我还是用checkfirst=True
ORMTable.__table__.create(bind=engine, checkfirst=True)
models.table
模型表
class TableA(Base):
class TableB(Base):
class NewTableC(Base):
id = Column('id', Text)
name = Column('name', Text)
form
形式
Then in the form action file:
然后在表单操作文件中：
engine = create_engine("sqlite:///myexample.db")
if not engine.dialect.has_table(engine, table_name):
# Added to models.tables the new table I needed ( format Table as written above )
table_models = importlib.import_module('models.tables')
# Grab the class that represents the new table
# table_name = 'NewTableC'
ORMTable = getattr(table_models, table_name)
# checkfirst=True to make sure it doesn't exists
ORMTable.__table__.create(bind=engine, checkfirst=True)
