How to perform a join_asof in Polars looking only backward and matching on specific conditions?

I'm trying to perform a join_asof in Polars where it only looks backward in time, ignoring matches on the same row. Specifically, I want it to find the last row where the Team and Field match exactly, but only if the Wk value is less than the current row's Wk.

The goal is to get the last GeneralLambda for recent games, either "Away" or "Home," excluding the current game. I thought the best way to do this would be with a join_asof, but I'm struggling with how to make it only consider past rows and match on both Team and Field.

Here is an example DataFrame to illustrate:

For each row, I need to join it with the last matching row where:

• Team is the same,
• Field is the same,
• Wk is less than the current row’s Wk.

I hope the DataFrame looks like this:

Is there a way to do this in Polars?

(
    df.sort("Wk")
    .with_columns(
        pl.col("GeneralLambda")
        .shift(1)
        .fill_null(pl.col("Team").str.tail(1) + "Last" + pl.col("Field") + "Value")
        .over("Team","Field")
        .alias("Prev_GeneralLambda")
    )
)

shape: (15, 6)
┌────────┬─────┬───────┬───────┬───────────────┬────────────────────┐
│ Season ┆ Wk  ┆ Team  ┆ Field ┆ GeneralLambda ┆ Prev_GeneralLambda │
│ ---    ┆ --- ┆ ---   ┆ ---   ┆ ---           ┆ ---                │
│ i64    ┆ i64 ┆ str   ┆ str   ┆ f64           ┆ str                │
╞════════╪═════╪═══════╪═══════╪═══════════════╪════════════════════╡
│ 2024   ┆ 25  ┆ TeamA ┆ Away  ┆ 2.123456      ┆ ALastAwayValue     │
│ 2024   ┆ 25  ┆ TeamB ┆ Home  ┆ 1.234567      ┆ BLastHomeValue     │
│ 2024   ┆ 25  ┆ TeamC ┆ Away  ┆ 0.987654      ┆ CLastAwayValue     │
│ 2024   ┆ 25  ┆ TeamD ┆ Home  ┆ 1.345678      ┆ DLastHomeValue     │
│ 2024   ┆ 25  ┆ TeamE ┆ Away  ┆ 1.456789      ┆ ELastAwayValue     │
│ …      ┆ …   ┆ …     ┆ …     ┆ …             ┆ …                  │
│ 2024   ┆ 27  ┆ TeamA ┆ Away  ┆ 2.278759      ┆ 2.123456           │
│ 2024   ┆ 27  ┆ TeamB ┆ Away  ┆ 1.103829      ┆ 1.345678           │
│ 2024   ┆ 27  ┆ TeamC ┆ Home  ┆ 0.992563      ┆ 0.876543           │
│ 2024   ┆ 27  ┆ TeamD ┆ Home  ┆ 1.089324      ┆ 1.345678           │
│ 2024   ┆ 27  ┆ TeamE ┆ Home  ┆ 1.074221      ┆ 1.56789            │
└────────┴─────┴───────┴───────┴───────────────┴────────────────────┘