I am trying to automate a complete application schema rebuild in Django. Basically, drop all the old tables, delete the migration and cache file(s), delete the migration history, and then makemigrations and migrate. I understand this will have some scratching their heads but I like to configuration control my test data and load what I need from csv files and "starting from scratch" is much simpler with this model.
I'm hopeful the automation will work against all the different Django-supported databases so trying to come up with an approach that's generic (although some gaps and brute-force).
As noted above, the first step is to drop all the existing tables. Under the assumption that most model changes aren't deletes, I'm doing some class introspection to identify all the models and then attempting to discern if the related tables exist:
app_models = get_all_app_models(models.Model, app_label)
# Loop through and remove models that no longer exist by doing a count, which should
# work no matter the SQL DBMS.
with connection.cursor() as cursor:
for app_model in app_models[:]:
try:
cursor.execute (f'select count (*) from {app_model._meta.db_table}')
except OperationalError:
app_models.remove(app_model)
<lots more code, doing cursor and other stuff that works OK>
Once the above completes, app_models contains tables that remain and the code then works through dropping them (which itself isn't trivial in a generic way).
The processing is contained in a Django view/form and once complete it attempts to render a simple "okey dokey" page. The problem is that an exception is thrown during the render saying "no such table: xxx" and it refers to the execute statement. There is no other call stack context displayed.
The table mentioned was indeed referenced in the execute, in fact it was the last one in the original app_models. Somehow it seems that Django is retaining error information after the cursor is closed (the 'with' cleanup) and somehow executing against it during the render processing. I tried to execute a "good" SQL statement to "clear" the error, but no luck. I tried to "del" the cursor, also no luck.
Update 3/25: After many hours of trial and error, I have found that if I place the above code (along with the rest of the processing) in a separate function and call it from the view/form function I no longer get the spurious call-back to the execute statement. I researched SQLite quite a bit and it seems that OperationalError is handled differently than (say) DataError. I tried other approaches to clearing the error (e.g. closing the connection) without avail. I suspect the web server is doing something tricky with the stack and is treating the OperationalError incorrectly and the function nesting "hides" it. Cheers.
I dont have much knowledge in dbs, but wanted to know if there is any technique by which when i update or insert a specific entry in a table, it should notify my python application to which i can then listen whats updated and then update that particular row, in the data stored in session or some temporary storage.
I need to send data filter and sort calls again n again, so i dont want to fetch whole data from sql, so i decided to keep it local, nd process it from there. But i was worried if in the mean time the db updates, and i could have been passing the same old data to filter requests.
Any suggestions?
rdbs only will be updated by your program's method or function sort of things.
you can just print console or log inside of yours.
if you want to track what updated modified deleted things,
you have to build a another program to able to track the logs for rdbs
thanks.
I am facing a strange problem right now. I am using pypyodbc to insert data into a test database hosted by AWS. This database that I created was by hand and did not imitate all relations and whatnot between tables. All I did was create a table with the same columns and the same datatypes as the original (let's call it master) database. When I run my code and insert the data it works in the test environment. Then I change it over to the master database and the code runs all the way through but no data is actually inputted. Is there any chance that there are security protocols in place which prevent me from inputting data in through the Python script rather than through a normal SQL query? Is there something I am missing?
It sounds like it's not pointing to the correct database. Have you made sure the connection information changes to point to the correct DB? So the server name is correct, the login credentials are good, etc.?
I'm looking for a way to constantly check my database (MySQL) for new entries. Once a new entry is committed I want to output it in a webpage using Flask.
Since the process takes time to finish I would like to give the users the impression it took only few seconds to retrieve data.
For now I'm waiting that the whole process finishes to give to the user the whole result. But I would prefer to update the result web-page every time a new entry was added to the DB. So for example the first entry is added to the DB, immediately the user can see it on the web-page, then a second entry is added the user can now see both the first and the second entries on the web-page and so on. I don't know if it has to come from flask or other ways
Any idea?
You can set MySQL to log all commits to General Query Log and monitor all changes (for example via Watchdog or PyNotify). Once the file changes, you can parse the new log entries and get the signal. By this way you'll avoid pooling for changes.
The better way would be of course send the signal while storing data to the database.
I have created a Python module that creates and populates several SQLite tables. Now, I want to use it in a program but I don't really know how to call it properly. All the tutorials I've found are essentially "inline", i.e. they walk through using SQLite in a linear fashion rather than how to actually use it in production.
What I'm trying to do is have a method check to see if the database is already created. If so, then I can use it. If not, an exception is raised and the program will create the database. (Or use if/else statements, whichever is better).
I created a test script to see if my logic is correct but it's not working. When I create the try statement, it just creates a new database rather than checking if one already exists. The next time I run the script, I get an error that the table already exists, even if I tried catching the exception. (I haven't used try/except before but figured this is a good time to learn).
Are there any good tutorials for using SQLite operationally or any suggestions on how to code this? I've looked through the pysqlite tutorial and others I found but they don't address this.
Don't make this more complex than it needs to be. The big, independent databases have complex setup and configuration requirements. SQLite is just a file you access with SQL, it's much simpler.
Do the following.
Add a table to your database for "Components" or "Versions" or "Configuration" or "Release" or something administrative like that.
CREATE TABLE REVISION(
RELEASE_NUMBER CHAR(20)
);
In your application, connect to your database normally.
Execute a simple query against the revision table. Here's what can happen.
The query fails to execute: your database doesn't exist, so execute a series of CREATE statements to build it.
The query succeeds but returns no rows or the release number is lower than expected: your database exists, but is out of date. You need to migrate from that release to the current release. Hopefully, you have a sequence of DROP, CREATE and ALTER statements to do this.
The query succeeds, and the release number is the expected value. Do nothing more, your database is configured correctly.
AFAIK an SQLITE database is just a file.
To check if the database exists, check for file existence.
When you open a SQLITE database it will automatically create one if the file that backs it up is not in place.
If you try and open a file as a sqlite3 database that is NOT a database, you will get this:
"sqlite3.DatabaseError: file is encrypted or is not a database"
so check to see if the file exists and also make sure to try and catch the exception in case the file is not a sqlite3 database
SQLite automatically creates the database file the first time you try to use it. The SQL statements for creating tables can use IF NOT EXISTS to make the commands only take effect if the table has not been created This way you don't need to check for the database's existence beforehand: SQLite can take care of that for you.
The main thing I would still be worried about is that executing CREATE TABLE IF EXISTS for every web transaction (say) would be inefficient; you can avoid that by having the program keep an (in-memory) variable saying whether it has created the database today, so it runs the CREATE TABLE script once per run. This would still allow for you to delete the database and start over during debugging.
As #diciu pointed out, the database file will be created by sqlite3.connect.
If you want to take a special action when the file is not there, you'll have to explicitly check for existance:
import os
import sqlite3
if not os.path.exists(mydb_path):
#create new DB, create table stocks
con = sqlite3.connect(mydb_path)
con.execute('''create table stocks
(date text, trans text, symbol text, qty real, price real)''')
else:
#use existing DB
con = sqlite3.connect(mydb_path)
...
Sqlite doesn't throw an exception if you create a new database with the same name, it will just connect to it. Since sqlite is a file based database, I suggest you just check for the existence of the file.
About your second problem, to check if a table has been already created, just catch the exception. An exception "sqlite3.OperationalError: table TEST already exists" is thrown if the table already exist.
import sqlite3
import os
database_name = "newdb.db"
if not os.path.isfile(database_name):
print "the database already exist"
db_connection = sqlite3.connect(database_name)
db_cursor = db_connection.cursor()
try:
db_cursor.execute('CREATE TABLE TEST (a INTEGER);')
except sqlite3.OperationalError, msg:
print msg
Doing SQL in overall is horrible in any language I've picked up. SQLalchemy has shown to be easiest from them to use because actual query and committing with it is so clean and absent from troubles.
Here's some basic steps on actually using sqlalchemy in your app, better details can be found from the documentation.
provide table definitions and create ORM-mappings
load database
ask it to create tables from the definitions (won't do so if they exist)
create session maker (optional)
create session
After creating a session, you can commit and query from the database.
See this solution at SourceForge which covers your question in a tutorial manner, with instructive source code :
y_serial.py module :: warehouse Python objects with SQLite
"Serialization + persistance :: in a few lines of code, compress and annotate Python objects into SQLite; then later retrieve them chronologically by keywords without any SQL. Most useful "standard" module for a database to store schema-less data."
http://yserial.sourceforge.net
Yes, I was nuking out the problem. All I needed to do was check for the file and catch the IOError if it didn't exist.
Thanks for all the other answers. They may come in handy in the future.