Building a psycopg2 query using a list of the column names to fetch - Debian

swiss_knight
July 16, 2021
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0 votes
2 Answers

A rather simple question but for which we surprisingly didn’t found a solution.

Here is my current code, for executing a simple SQL query on a PostgreSQL database from Python 3.6.9 using psycopg2 ('2.9.1 (dt dec pq3 ext lo64)'):

import psycopg2

myid = 100
fields = ('p.id', 'p.name', 'p.type', 'p.price', 'p.warehouse', 'p.location', )
sql_query = ("SELECT " + ', '.join(fields) + " FROM product p "
              "INNER JOIN owner o ON p.id = o.product_id "
              "WHERE p.id = {} AND (o.dateof_purchase IS NOT NULL "
              "OR o.state = 'checked_out' );"
        ).format(myid)

try:
    with psycopg2.connect(**DB_PARAMS) as conn:
        with conn.cursor(cursor_factory=DictCursor) as curs:
            curs.execute(sql_query, )
            row = curs.fetchone()

except psycopg2.Error as error:
    raise ValueError(f"ERR: something went wrong with the query :n{sql_query}") from None

We’re more and more thinking that this is… not good. (awfully bad to be honest).

Therefore, we’re trying to use a modern f-string notation:

sql_query = (f"""SELECT {fields} FROM product p
             INNER JOIN owner o ON p.id = o.product_id
             WHERE p.id = {myid} AND (o.dateof_purchase IS NOT NULL
             OR o.state = 'checked_out' );""")

But then, the query looks like:

SELECT  ('p.id', 'p.name', 'p.type', 'p.price', 'p.warehouse', 'p.location', ) FROM ...;

which is not valid in PSQL because 1. of the brackets, and 2. of the single quoted column names.

We’d like to figure out a way to get rid of these.

In between, we went back to the doc and remembered this:

https://www.psycopg.org/docs/usage.html

Ooops! So we refactored it this way:

sql_query = (f"""SELECT %s FROM product p
             INNER JOIN owner o ON p.id = o.product_id
             WHERE p.id = %s AND (o.dateof_purchase IS NOT NULL
             OR o.state = 'checked_out' );""")  

try:
    with psycopg2.connect(**DB_PARAMS) as conn:
        with conn.cursor(cursor_factory=DictCursor) as curs:
            # passing a tuple as it only accept one more argument after the query!
            curs.execute(sql_query, (fields, myid))
            row = curs.fetchone()

and mogrify() says:

"SELECT ('p.id', 'p.name', 'p.type', 'p.price', 'p.warehouse', 'p.location', ) FROM ...;"

here again, the brackets and the single quotes are causing troubles, but no error is actually raised.
The only thing is that row evaluates to this strange result:

['('p.id', 'p.name', 'p.type', 'p.price', 'p.warehouse', 'p.location', )']

So, how could we cleverly and dynamically build a psycopg2 query using a list of parameters for column names without neglecting the security?

(A trick could be to fetch all columns and filter them out after… but there are too many columns, some with quiet large amount of data that we don’t need, that’s why we want to run a query using a precisely defined selection of columns, which may get dynamically extended by some function, otherwise we would have hard-coded these column names of course).

OS: Ubuntu 18.04
PostgreSQL: 13.3 (Debian 13.3-1.pgdg100+1)

Answers

Chosen as BEST ANSWER

I finally found a solution. It makes use of map to use a list or a tuple of column names and sql.Literal to use a given id, this is maybe cleaner:

conn = psycopg2.connect(**DB_PARAMS)

myid = 100
# using the simple column identifiers
fields_1 = ('id', 'name', 'type', 'price', 'warehouse', 'location',)
# using the dot notation with the table alias 'p' as the prefix:
fields_2 = ('p.id', 'p.name', 'p.type', 'p.price', 'p.warehouse', 'p.location',)

sql_query_1 = sql.SQL("""
    SELECT {f} FROM product p
    INNER JOIN owner o ON p.id = o.product_id
    WHERE p.id = {j} AND (o.dateof_purchase IS NOT NULL
    OR o.state = 'checked_out' );"""
).format(
    f = sql.SQL(',').join(map(sql.Identifier, fields_1)),
    j = sql.Literal(myid)
)

sql_query_2 = sql.SQL("""
    SELECT {f} FROM product p
    INNER JOIN owner o ON p.id = o.product_id
    WHERE p.id = {j} AND (o.dateof_purchase IS NOT NULL
    OR o.state = 'checked_out' );"""
).format(
    f = sql.SQL(',').join(map(sql.SQL, fields_2)), # use sql.SQL!
    j = sql.Literal(myid)
)

sql_query_2b = sql.SQL("""
    SELECT {f} FROM product p
    INNER JOIN owner o ON p.id = o.product_id
    WHERE p.id = {j} AND (o.dateof_purchase IS NOT NULL
    OR o.state = 'checked_out' );"""
).format(
    f = sql.SQL(',').join(map(sql.Identifier, fields_2)), # DON'T use sql.Identifier!
    j = sql.Literal(myid)
)

# VALID SQL QUERY:
print(sql_query_1.as_string(conn))
# will print:
# SELECT "id","name","type","price","warehouse","location" FROM product p
#    INNER JOIN owner o ON p.id = o.product_id
#    WHERE p.id = 100 AND (o.dateof_purchase IS NOT NULL
#    OR o.state = 'checked_out' );


# VALID SQL QUERY:
print(sql_query_2.as_string(conn))
# will print:
# SELECT p.id,p.name,p.type,p.price,p.warehouse,p.location FROM product p
#    INNER JOIN owner o ON p.id = o.product_id
#    WHERE p.id = 100 AND (o.dateof_purchase IS NOT NULL
#    OR o.state = 'checked_out' );

# /! INVALID SQL QUERY /!:
print(sql_query_2b.as_string(conn))
# will print:
# SELECT "p.id","p.name","p.type","p.price","p.warehouse","p.location" FROM product p
#    INNER JOIN owner o ON p.id = o.product_id
#    WHERE p.id = 100 AND (o.dateof_purchase IS NOT NULL
#    OR o.state = 'checked_out' );

But because of that:

simple columns names are evaluated correctly when in double quotes, eg. id is equivalent to "id", name is equivalent to "name" for PostgreSQL,
column name, when prefixed with the dot notation using the table alias or identifier, e.g. p.id or product.id instead of just id or "id" will miserably fail with the following error:

UndefinedColumn: column "p.id" does not exist
LINE 1: SELECT "p.id","p.type","p.price","p.warehouse","p.location",...
               ^
HINT:  Perhaps you meant to reference the column "p.id".

(Edit)

The ‘%s’ insertion will try to turn every argument into an SQL string, as @AdamKG pointed out. Instead, you can use the psycopg2.sql module will allow you to insert identifiers into queries, not just strings:

from psycopg2 import sql

fields = ('id', 'name', 'type', 'price', 'warehouse', 'location', )

sql_query = sql.SQL(
          """SELECT {} FROM product p
             INNER JOIN owner o ON p.id = o.product_id
             WHERE p.id = %s AND (o.dateof_purchase IS NOT NULL
             OR o.state = 'checked_out' );""")

try:
    with psycopg2.connect(**DB_PARAMS) as conn:
        with conn.cursor(cursor_factory=DictCursor) as curs:
            # passing a tuple as it only accept one more argument after the query!
            curs.execute(sql_query.format(*[sql.Identifier(field) for field in fields]), (*fields, myid))
            row = curs.fetchone()

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Building a psycopg2 query using a list of the column names to fetch – Debian

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