I have researched this topic and have decided just to ask here since I can't seem to find anything. I'll explain below:
Context: Flask Application with a form the client fills out and posts to the server. The form inputs are used to create a query and return data.
I am using SQLalchemy currently to construct the query from scratch. At this point, I have successfully connected to my existing Redshift database and can query properly but I cannot figure out how to dynamically construct a simple Select x, y, z statement based on the user's form inputs.
The main problem being that Query() can't take in a python list of columns. It seems you must specify each column like table.c.column1 which doesn't work well with a dynamic query since I don't know what columns I want until the user submits the form.
My 2 ideas so far:
Loop through all column names and use Query.add_columns(table.c['colname'])
Use select([col1, col2, ...]) instead of Query()
Use load_columns() to load only specific columns in a table to query. Unfortunately seems to only work with model objects and not reflected tables unless I am mistaken
Both of these seem backwards to me as they do not really accomplish my goal effectively.
SQLAlchemy is quite flexible, so both 1 and 2 get the job done. If you've no need for ORM functionality, then perhaps #2 is more natural. If the user were to pass a list of column names such as
columns = request.args.getlist('columns')
you could then create your select() quite easily with a bunch of column() constructs:
stmt = select([column(c) for c in columns]).\
select_from(some_table)
or if you have the table at hand, like you hint in the question:
stmt = select([table.c[c] for c in columns])
and then all that is left is to execute your statement:
results = db.session.execute(stmt).fetchall()
Related
I need to create a column in a table that is autoupdated if one or more columns (possibly in another table) are updated, but it also should be possible to edit this column directly (and value should be kept in sql unless said other cols are updated, in which case first logic is applied)
I tried column_property but it seems that its merely a construction inside python and doesnt represent an actual column
I also tried hybrid_property and default, both didnt accomplish this
This looks like insert/update trigger, however i want to know "elegant" way to declare it if its even possible
I use declarative style for tables on postgres
I dont make any updates to sql outside of sqlalchemy
Definitely looks like insert/update triggers. But if I were you, I would incapsulate this logic in python by using 2 queries , so it will be more clear
I am currently creating a web app in Flask and use SQL-alchemy (not the flask version) to deal with reading and writing to my MySQL database.
I have about 15 different tables each mapped to a different declarative class, however the application is still in beta stages and so this number will probably increase.
I would like a way to iterate through every single table and run the same command on every single one. This is part of an update function where an admin can change the name of a book, this name change should be reflected in all the other tables where that book is referred to.
Is there a way to iterate through all your SqlAlchemy tables?
Thanks!
Not exactly sure what you want to achieve here, but if you use declarative base, you can try something like this:
tables = Base.__subclasses__()
for t in tables:
rows = Session.query(t).all()
for r in rows:
... do something ...
This gets all tables by listing subclasses of Base. Then it queries everything from each table in turn and loops through selected rows.
However, I do not quite understand why you would want to do this. How you describe your question is that you should have a Book table, and all others link to it if they want to reference books. This would be the relational model instead of dragging information on Books in each and every table and trying to manage them like this manually.
Problem.
I have query let say like that:
qr = Session.query(models.User).filter(models.User.email == email)
What I want to do is to create qr2 with all filters dropped, in my case it would equivalent to:
qr2 = Session.query(models.User)
Good solution might be either, new query based on old one but without filters or modification of old query.
Why someone might need that.
Imagine you have a complex query that was build 3 modules and 4k lines away. What you get in your tiny function is query object (an instance). You have no clue which models are being selected or in fact what is the real payload of the query. All you know that you want to modify that query to not have any filters at all.
Suggestions/Solutions?
I'm in the processing of moving over a mysql database to a postgres database. I have read all of the articles presented here, as well as reading over some of the solutions presented on stackoverflow. The tools recommended don't seem to work for me. Both databases were generated by Django's syncdb, although the postgres db is more or less empty at the moment. I tried to migrate the tables over using Django's built in dumpdata / loaddata functions and its serializers, but it doesn't seem to like a lot of my tables, leading me to believe that writing a manual solution might be best in this case. I have code to verify that the column headers are the same for each table in the database and that the matching tables exist- that works fine. I was thinking it would be best to just grab the mysql data row by row and then insert it into the respective postgres table row by row (I'm not concerned with speed atm). The one thing is, I don't know what's the proper way to construct the insert statement. I have something like:
table_name = retrieve_table()
column_headers = get_headers(table_name) #can return a tuple or a list
postgres_cursor = postgres_con.cursor()
rows = mysql_cursor.fetchall()
for row in rows: #row is a tuple
postgres_cursor.execute(????)
Where ??? would be the insert statement. I just don't know what the proper way is to construct it. I have the table name that I would like to insert into as a string, I have the column headers that I can treat as a list, tuple, or string, and I have the respective values that I'd like to insert. What would be the recommended way to construct the statement? I have read the documentation on psycopg's documentation page and I didn't quite see the way that would satisfy my needs. I don't know (or think) this is the entirely correct way to properly migrate, so if someone could steer me in the correct way or offer any advice I'd really appreciate it.
Two questions:
i want to generate a View in my PostGIS-DB. How do i add this View to my geometry_columns Table?
What i have to do, to use a View with SQLAlchemy? Is there a difference between a Table and View to SQLAlchemy or could i use the same way to use a View as i do to use a Table?
sorry for my poor english.
If there a questions about my question, please feel free to ask so i can try to explain it in another way maybe :)
Nico
Table objects in SQLAlchemy have two roles. They can be used to issue DDL commands to create the table in the database. But their main purpose is to describe the columns and types of tabular data that can be selected from and inserted to.
If you only want to select, then a view looks to SQLAlchemy exactly like a regular table. It's enough to describe the view as a Table with the columns that interest you (you don't even need to describe all of the columns). If you want to use the ORM you'll need to declare for SQLAlchemy that some combination of the columns can be used as the primary key (anything that's unique will do). Declaring some columns as foreign keys will also make it easier to set up any relations. If you don't issue create for that Table object, then it is just metadata for SQLAlchemy to know how to query the database.
If you also want to insert to the view, then you'll need to create PostgreSQL rules or triggers on the view that redirect the writes to the correct location. I'm not aware of a good usage recipe to redirect writes on the Python side.