问题
I have a flask-sqlalchemy model:
class MyModel(db.Model):
__tablename__ = 'targets'
id = db.Column(db.Integer, primary_key=True)
url = db.Column(db.String(2048))
The table has already been created, and is in use. I want to create an index on the url attribute, so I pass index=True to it:
url = db.Column(db.String(2048), index=True)
How can I make this index take effect, without deleting and recreating the table?
回答1:
Given the model class from the original question.
class MyModel(db.Model):
__tablename__ = 'targets'
id = db.Column(db.Integer, primary_key=True)
url = db.Column(db.String(2048))
You cannot just add index=True because even if you called db.Model.metadata.create_all() the index will not be created on an already created table.
Instead, you need to create an independent Index object, and then create it. It will look something like this:
class MyModel(db.Model):
__tablename__ = 'targets'
id = db.Column(db.Integer, primary_key=True)
url = db.Column(db.String(2048))
mymodel_url_index = Index('mymodel_url_idx', MyModel.url)
if __name__ == '__main__':
mymodel_url_index.create(bind=engine)
Now where engine comes from will be up to your sqlalchemy configuration, but this code should convey the gist of what needs to happen.
回答2:
If you're using Alembic, you can create a migration to do this, and avoid adding it into the model at all:
def upgrade():
# ### commands auto generated by Alembic - please adjust! ###
op.create_index('table1_id', 'table1', ['id'], unique=True)
op.create_index('table2_id', 'table2', ['id'], unique=True)
# ### end Alembic commands ###
def downgrade():
# ### commands auto generated by Alembic - please adjust! ###
op.drop_index('table1_id', table_name='table1')
op.drop_index('table2_id', table_name='table2')
# ### end Alembic commands ###
回答3:
Call create() on the Index:
index.create()
http://docs.sqlalchemy.org/en/latest/core/constraints.html#sqlalchemy.schema.Index.create
回答4:
Since the question was asked, support has been added for this.
Now you can just add index=True to an existing column, and auto-generate the migration.
Checked on the following package versions:
alembic==1.0.10
SQLAlchemy==1.3.4
SQLAlchemy-Utils==0.34.0
Flask-SQLAlchemy==2.4.0
Flask-Migrate==2.5.2
回答5:
Please note that this is incorrect and over-complicated answer
The right way is to use index.create as it was said here.
First of all make sure that you have latest snapshot of your database and is able to restore database from this snapshot.
For medium and large size projects (the ones that you might need to support several versions at the same time and are installed on multiple environments) there is special procedure which is part of database management lifecycle called "database migration". DB migrations includes changes to existing schema. SQLAlchemy doesn't support migration out of the box.
There are two SQLAlchemy compatible database migration tools available:
- Alembic
- SQLAlchemy-Migrate
See more information and links to these tools in SQLAlchemy documentation page: Altering Schemas through Migrations.
But if your are working on small project I would suggest to manually run ALTER TABLE DDL query from the database command line utility or through connection.execute() in python script.
In the production application I'm working at right now, we support only one latest version of application. For every database schema change we do the following steps:
- make a snapshot of the production database
- load this snapshot on development environment
- update sqlalchemy data model module
- prepare and run alter table query and save this query for later
- make other related changes to the code
- run tests on dev environment
- deploy latest version of the code to production
- do alter table on production
Also I'm using the following trick for generating create table/index queries:
I point my application to brand new database, enable logging of sqlalchemy queries and run metadata.create_all() - so in logs (or STDOUT) I see create query generated by sqlalchemy
Depending on the database system you are using index creation query will be little different. Generic query would look like this:
create index targets_i on targets(url);
回答6:
I am not sure if this conforms to best practices but I found Alembic would notify me of Indexes in the __table_args__ but not actually make them for me during migrations. I made this small script that can generate new indexes found in the __table_args__ property. It makes use of Index.create() as mentioned above, but will generate new indexes if they do not exist.
def create_indexes(db, drop_index=False):
"""
Creates all indexes on models in project if they do not exists already. Assumes all models
inherit from RequiredFields class, otherwise will need to adjust search for subclasses. If the index
exists SQLAlchemy throws an error and we assume everything is ok.
:param db: The app db object, acts as the engine param for the Index.create()
:param drop_index: specifies whether the indexes should be dropped or created
:return:
"""
from application.base_models import RequiredFields
from sqlalchemy import Index
from sqlalchemy.exc import ProgrammingError
for klass in RequiredFields.__subclasses__():
if hasattr(klass, '__table_args__'):
for item in getattr(klass, '__table_args__'):
if isinstance(item, Index):
try:
if not drop_index:
item.create(db.engine)
else:
item.drop(db.engine)
except ProgrammingError: # If index exists, creation fails on error
pass
return
Here is a sample class showing the indexes.
class MyModel(RequiredFields):
__table_args__ = (
db.Index( ... ),
db.Index( ... ),
)
回答7:
Use flask-migrate.It's cool. After you add the index,just use this command:
python manage.py db migrate
Everything works fine
来源：https://stackoverflow.com/questions/14419299/adding-indexes-to-sqlalchemy-models-after-table-creation