feat(polars): add maintain-order flag to polars group-by and allow expression inputs in polars filter (#15865)

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# Description
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This PR involves two changes: (1) adding `maintain-order` flag to
`polars group-by` for stable sorting when aggregating and (2) allow
expression inputs in `polars filter`. The first change was necessary to
reliably test the second change, and the two commits are therefore
combined in one PR. See example:


```nushell
#  Filter a single column in a group-by context
  > [[a b]; [foo 1] [foo 2] [foo 3] [bar 2] [bar 3] [bar 4]] | polars into-df
                    | polars group-by a --maintain-order
                    | polars agg {
                        lt: (polars col b | polars filter ((polars col b) < 2) | polars sum)
                        gte: (polars col b | polars filter ((polars col b) >= 3) | polars sum)
                    }
                    | polars collect
  ╭───┬─────┬────┬─────╮
  │ # │  a  │ lt │ gte │
  ├───┼─────┼────┼─────┤
  │ 0 │ foo │  1 │   3 │
  │ 1 │ bar │  0 │   7 │
  ╰───┴─────┴────┴─────╯

```

# User-Facing Changes
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helps us keep track of breaking changes. -->
No breaking changes.

# Tests + Formatting
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An example test was added to `polars filter` demonstrating both the
stable group-by feature and the expression filtering feature.

# After Submitting
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crates/nu_plugin_polars/src/dataframe/command/aggregation/groupby.rs

@@ -31,6 +31,10 @@ impl PluginCommand for ToLazyGroupBy {
                 SyntaxShape::Any,
                 "Expression(s) that define the lazy group-by",
             )
+            .switch(
+                "maintain-order",
+                "Ensure that the order of the groups is consistent with the input data. This is slower than a default group by and cannot be run on the streaming engine.",
+                Some('m'))
             .input_output_type(
                 Type::Custom("dataframe".into()),
                 Type::Custom("dataframe".into()),
@@ -104,6 +108,7 @@ impl PluginCommand for ToLazyGroupBy {
         let vals: Vec<Value> = call.rest(0)?;
         let expr_value = Value::list(vals, call.head);
         let expressions = NuExpression::extract_exprs(plugin, expr_value)?;
+        let maintain_order = call.has_flag("maintain-order")?;
 
         if expressions
             .iter()
@@ -118,7 +123,7 @@ impl PluginCommand for ToLazyGroupBy {
 
         let pipeline_value = input.into_value(call.head)?;
         let lazy = NuLazyFrame::try_from_value_coerce(plugin, &pipeline_value)?;
-        command(plugin, engine, call, lazy, expressions)
+        command(plugin, engine, call, lazy, expressions, maintain_order)
             .map_err(LabeledError::from)
             .map(|pd| pd.set_metadata(metadata))
     }
@@ -130,8 +135,13 @@ fn command(
     call: &EvaluatedCall,
     mut lazy: NuLazyFrame,
     expressions: Vec<Expr>,
+    maintain_order: bool,
 ) -> Result<PipelineData, ShellError> {
-    let group_by = lazy.to_polars().group_by(expressions);
+    let group_by = if maintain_order {
+        lazy.to_polars().group_by_stable(expressions)
+    } else {
+        lazy.to_polars().group_by(expressions)
+    };
     let group_by = NuLazyGroupBy::new(group_by, lazy.from_eager, lazy.schema().clone()?);
     group_by.to_pipeline_data(plugin, engine, call.head)
 }

crates/nu_plugin_polars/src/dataframe/command/data/filter.rs

@@ -1,7 +1,7 @@
 use crate::{
     PolarsPlugin,
     dataframe::values::{Column, NuDataFrame, NuExpression, NuLazyFrame},
-    values::CustomValueSupport,
+    values::{CustomValueSupport, PolarsPluginObject, PolarsPluginType, cant_convert_err},
 };
 
 use nu_plugin::{EngineInterface, EvaluatedCall, PluginCommand};
@@ -31,10 +31,16 @@ impl PluginCommand for LazyFilter {
                 SyntaxShape::Any,
                 "Expression that define the column selection",
             )
-            .input_output_type(
+            .input_output_types(vec![
+                (
                     Type::Custom("dataframe".into()),
                     Type::Custom("dataframe".into()),
-            )
+                ),
+                (
+                    Type::Custom("expression".into()),
+                    Type::Custom("expression".into()),
+                ),
+            ])
             .category(Category::Custom("lazyframe".into()))
     }
 
@@ -99,6 +105,37 @@ impl PluginCommand for LazyFilter {
                     .into_value(Span::test_data()),
                 ),
             },
+            Example {
+                description: "Filter a single column in a group-by context",
+                example: "[[a b]; [foo 1] [foo 2] [foo 3] [bar 2] [bar 3] [bar 4]] | polars into-df
+                    | polars group-by a --maintain-order
+                    | polars agg {
+                        lt: (polars col b | polars filter ((polars col b) < 2) | polars sum)
+                        gte: (polars col b | polars filter ((polars col b) >= 3) | polars sum)
+                    }
+                    | polars collect",
+                result: Some(
+                    NuDataFrame::try_from_columns(
+                        vec![
+                            Column::new(
+                                "a".to_string(),
+                                vec![Value::test_string("foo"), Value::test_string("bar")],
+                            ),
+                            Column::new(
+                                "lt".to_string(),
+                                vec![Value::test_int(1), Value::test_int(0)],
+                            ),
+                            Column::new(
+                                "gte".to_string(),
+                                vec![Value::test_int(3), Value::test_int(7)],
+                            ),
+                        ],
+                        None,
+                    )
+                    .expect("simple df for test should not fail")
+                    .into_value(Span::test_data()),
+                ),
+            },
         ]
     }
 
@@ -113,8 +150,29 @@ impl PluginCommand for LazyFilter {
         let expr_value: Value = call.req(0)?;
         let filter_expr = NuExpression::try_from_value(plugin, &expr_value)?;
         let pipeline_value = input.into_value(call.head)?;
-        let lazy = NuLazyFrame::try_from_value_coerce(plugin, &pipeline_value)?;
+
+        match PolarsPluginObject::try_from_value(plugin, &pipeline_value)? {
+            PolarsPluginObject::NuDataFrame(df) => {
+                command(plugin, engine, call, df.lazy(), filter_expr)
+            }
+            PolarsPluginObject::NuLazyFrame(lazy) => {
                 command(plugin, engine, call, lazy, filter_expr)
+            }
+
+            PolarsPluginObject::NuExpression(expr) => {
+                let res: NuExpression = expr.into_polars().filter(filter_expr.into_polars()).into();
+                res.to_pipeline_data(plugin, engine, call.head)
+            }
+
+            _ => Err(cant_convert_err(
+                &pipeline_value,
+                &[
+                    // PolarsPluginType::NuDataFrame,
+                    PolarsPluginType::NuLazyGroupBy,
+                    PolarsPluginType::NuExpression,
+                ],
+            )),
+        }
         .map_err(LabeledError::from)
         .map(|pd| pd.set_metadata(metadata))
     }