chore: fix field bug

datafusion/functions/src/core/expr_ext.rs

@@ -0,0 +1,68 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! Extension methods for Expr.
+
+use datafusion_expr::{Expr, Literal};
+
+use super::expr_fn::get_field;
+
+pub trait FieldAccessor {
+    fn field(self, name: impl Literal) -> Expr;
+}
+
+impl FieldAccessor for Expr {
+    /// Return access to the named field. Example `expr["name"]`
+    ///
+    /// ## Access field "my_field" from column "c1"
+    ///
+    /// For example if column "c1" holds documents like this
+    ///
+    /// ```json
+    /// {
+    ///   "my_field": 123.34,
+    ///   "other_field": "Boston",
+    /// }
+    /// ```
+    ///
+    /// You can access column "my_field" with
+    ///
+    /// ```
+    /// # use datafusion_expr::{col};
+    /// # use datafusion_functions::core::expr_ext::FieldAccessor;
+    /// let expr = col("c1")
+    ///    .field("my_field");
+    /// assert_eq!(expr.display_name().unwrap(), "c1[my_field]");
+    /// ```
+    fn field(self, name: impl Literal) -> Expr {
+        get_field(self, name)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use datafusion_expr::col;
+
+    #[test]
+    fn test_field() {
+        let expr1 = col("a").field("b");
+        let expr2 = get_field(col("a"), "b");
+        assert_eq!(expr1, expr2);
+    }
+}