Python script for importing .csv to_sql with Pandas fails unless I DROP TRIGGER for updating materialized viewb

I have a Postgres database with a table with a materialized view that automatically updates every time changes are made to the table via the following user-defined function and trigger:

I have a Postgres database with a table with a materialized view that automatically updates every time changes are made to the table via the following user-defined function and trigger:

create function refresh_matview_dohscrapemat()
returns trigger language plpgsql
as $$
begin
    refresh materialized view dohscrape;
    return null;
end $$;
create trigger refresh_matview_dohscrapemat
after insert or update or delete or truncate
on dohscrape for each statement
execute procedure refresh_matview_dohscrapemat();

I have a python script that uses pandas to automatically import .csv files into this table that only works if I DROP the TRIGGER for updating the materialized view before running it.

If I try to run the script without dropping the trigger I get the following error:

sqlalchemy.exc.NotSupportedError: (psycopg2.NotSupportedError) "dohscrape" is not a materialized view CONTEXT: SQL statement "refresh materialized view dohscrape" PL/pgSQL function refresh_matview_dohscrapemat() line 3 at SQL statement [SQL: 'INSERT INTO dohscrape (filename, content) VALUES (%(filename)s, %(content)s)'] [parameters: ({'filename':...(Background on this error at: http://sqlalche.me/e/tw8g)

It says "dohscrape" is not a materialized view, which is correct because that's the name of the table not the materialized view. The materialized view is named "dohscrapemat".

The link in the error message points to the following info:

NotSupportedError Exception raised in case a method or database API was used which is not supported by the database, e.g. requesting a .rollback() on a connection that does not support transaction or has transactions turned off.
This error is a DBAPI Error and originates from the database driver (DBAPI), not SQLAlchemy itself.

But I don't see how this would relate to an error caused by/related to a trigger for updating a materialized view, which I assume is the root issue because dropping it resolves the error.

Once I let the script upload to the table and recreate the trigger it all works fine, but I'd like to be able to run this script without having to drop and recreate the trigger.

Why is the trigger for refreshing the materialized view causing an import error? And why is pandas/sqlalchemy/psycopg2 confuse my table with its materialized view?

The snippet of code in the python script that uploads to the db is:

for files in source:
    if files.endswith(".csv"):
        df = pd.read_csv(os.path.join(sourcepath,files))
        df.to_sql(name='dohscrape',con=dbconn,if_exists='append',index=False)

I'm using Python 3.7 and Postgres 11.

Using PostgreSQL Database with Python

Using PostgreSQL Database with Python

In this article we will see how to connect to PostgreSQL from Python Script and perform queries.

In this article we will see how to connect to PostgreSQL from Python Script and perform queries.

PostgreSQL is an open source object-relational database management system. PostgreSQL is ACID-compliant and is transactional. It has triggers, foreign keys and supports functions and stored procedures.

PostgreSQL is used by giants like Uber, Apple, Netflix and Instagram.

Requirement:

Create a virtual environment using python 3 and activate it. Install below packages.

psycopg2==2.7.3.2

Installation:

Install the PostgreSQL database and utilities using below commands.

sudo apt-get update
sudo apt-get install postgresql postgresql-contrib

By default, PostgreSQL sets up the user and database “postgres” upon a new installation. We need to switch to this user to use postgres database.

sudo -su postgres

Now go to the Postgres prompt by typing psql on terminal.

We are using version 10.3.

If you get any error in connecting to database, make sure PostgreSQL is running. Check the status using below command.

$ systemctl status postgresql

You can check for errors in logs using below command.

$ tail -f /var/log/postgresql

Creating database:

Before creating a new database, lets list all the databases. Use \l or \list for the same.

To create database, exit the psql terminal by typing \q and use command createdb testdb.

[email protected]:~$ createdb testdb
[email protected]:~$ psql
psql (10.3 (Ubuntu 10.3-1.pgdg16.04+1))
Type "help" for help.

postgres=# \l
                               List of databases
     Name      |  Owner   | Encoding | Collate | Ctype |   Access privileges   
---------------+----------+----------+---------+-------+-----------------------
 postgres      | postgres | UTF8     | en_IN   | en_IN | 
 rana_test     | postgres | UTF8     | en_IN   | en_IN | 
 template0     | postgres | UTF8     | en_IN   | en_IN | =c/postgres          +
               |          |          |         |       | postgres=CTc/postgres
 template1     | postgres | UTF8     | en_IN   | en_IN | =c/postgres          +
               |          |          |         |       | postgres=CTc/postgres
 testdb        | postgres | UTF8     | en_IN   | en_IN | 
(5 rows)

postgres=# \c testdb
You are now connected to database "testdb" as user "postgres".
testdb=# 

To connect to another database, use command \c or \connect and database name. \c testdb in this case.

Creating Table:

Most the query sytanx in PostgreSQL are same as MySQL.

create table users (
    id serial PRIMARY KEY,
    username varchar (20) NOT NULL,
    age smallint NOT NULL,
    location varchar (50) NOT NULL
);

Copy paste the above sytax in terminal and new table will be created. You can list the tables by typing \d.

testdb=# create table users (
testdb(#     username varchar (20) NOT NULL,
testdb(#     age smallint NOT NULL,
testdb(#     location varchar (50) NOT NULL
testdb(# );
CREATE TABLE
testdb=# \d
         List of relations
 Schema | Name  | Type  |  Owner   
--------+-------+-------+----------
 public | users | table | postgres
(1 row)

testdb=# 

You can learn more about querying from psql terminal by visitng official site. Lets go to Python code.

Connecting from Python Script:

We installed the psycopg package in virtual environment. Use below code in Python Script to connect to database.

import psycopg2


# this function will return the connection object
def connect():
    conn = None
    try:
        conn = psycopg2.connect(host="localhost", user="postgres", password="root", database="testdb")
    except Exception as e:
        print(repr(e))

    return conn

Inserting Data into Table:

First get the connection and cursor and then create query. Once query is executed, commit using connection and close the cursor and connection.

conn = connect()
cur = conn.cursor()

last_insert_id = None

# inserting data in users table
sql_query = "insert into users (username, age, location) values (%s, %s, %s) returning id;"

sql_data = (
    "Ajay",
    "25",
    "New York"
)


cur.execute(sql_query, sql_data)
last_insert_id = cur.fetchone()[0]
print("Last Insert ID " + str(last_insert_id))

conn.commit()
cur.close()
conn.close()

return last_insert_id

We are Inserting data in table and returning the primary key id which is the serial key.

Fetching Data from Table:

Select query for PostgreSQL is same as MySQL.

conn = connect()
cur = conn.cursor()

sql_query = "select username, age, location from users where location = %s;"
sql_data = ("Delhi")
cur.execute(sql_query, sql_data)

results = cur.fetchall()
return results

Updating a row:

conn = connect()
cursor = conn.cursor()

sql_query = "update users set location = %s where username = %s;"
sql_data = ("Mumbai", "Ajay")

cursor.execute(sql_query, sql_data)

cursor.close()
conn.close()

return True

To exit the terminal use \q command.

If you are facing any issue, feel free to comment.

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