Ambigiuous column names in join_where require post-join names

[ion-elgreco]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import datetime
import random
import polars as pl

def random_date(start, end):
    """Generate a random datetime between `start` and `end`"""
    return start + datetime.timedelta(
        # Get a random amount of seconds between `start` and `end`
        seconds=random.randint(0, int((end - start).total_seconds())),
    )

df = pl.DataFrame({
    "id":list(range(0,1000))*1500,
    "start_date": [random_date(datetime.datetime(2015,1,1), datetime.datetime(2020,1,1),) for i in range(1500000)]
}).with_columns(end_date = pl.col('start_date') + pl.duration(hours=random.randint(24,240)))

parts = df.sample(1700).select(
    'id',
    pl.concat_list(pl.col('start_date','end_date')).list.mean().alias('date').cast(pl.Datetime)
)

df.lazy().join_where(parts.lazy(), pl.col('id') == pl.col('id'),
    pl.col('start_date') <= pl.col('date'),
    pl.col('end_date') >= pl.col('date'))

Log output

No response

Issue description

This one tripped me for a sec, we need to refer within the col name to which side it is. Can't we have an expr for this?

Expected behavior

Just a thought but perhaps, pl.col('id').lhs() or pl.col('id').rhs()

Installed versions

--------Version info---------
Polars:              1.7.1
Index type:          UInt32
Platform:            Linux-5.15.153.1-microsoft-standard-WSL2-x86_64-with-glibc2.39
Python:              3.10.14 (main, Aug 14 2024, 05:11:29) [Clang 18.1.8 ]

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            0.19.1
fastexcel            <not installed>
fsspec               <not installed>
gevent               <not installed>
great_tables         <not installed>
matplotlib           <not installed>
nest_asyncio         1.6.0
numpy                1.22.2
openpyxl             <not installed>
pandas               <not installed>
pyarrow              17.0.0
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>

[cmdlineluser]

Ritchie did mention this on Discord:

We could add a pl.col_left,pl.col_right ... 🤔

[AlexeyDmitriev]

Note, semantics were changed here: #18640 (comment)