How to split a polars DataFrame into several dataframes in rust?

I use Polars to read from a CSV file. I want to separate the DataFrame into several DataFrames by the "date" column (I want to analyze the data by day).

I tried to use partition_by but it seems not to be implemented (I got the error: "no method named partition_by found for mutable reference &mut DataFrame in the current scopemethod not found in &mut DataFrame").