Implement toString connectors method

apollo-federation/src/connectors/json_selection/helpers.rs

@@ -79,6 +79,19 @@ pub(crate) const fn json_type_name(v: &JSON) -> &str {
     }
 }
 
+/// Provides a standard method to convert JSON to string.
+/// Errors on arrays or objects because "stringigying" is not semantically the same as converting to a string.
+/// null is returned as None but commonly, it gets converted to a blank string ("")
+pub(crate) fn json_to_string(json: &JSON) -> Result<Option<String>, &'static str> {
+    match json {
+        JSON::Null => Ok(None),
+        JSON::Bool(b) => Ok(Some(b.to_string())),
+        JSON::Number(n) => Ok(Some(n.to_string())),
+        JSON::String(s) => Ok(Some(s.as_str().to_string())),
+        JSON::Array(_) | JSON::Object(_) => Err("cannot convert arrays or objects to strings."),
+    }
+}
+
 pub(crate) fn vec_push<T>(mut vec: Vec<T>, item: T) -> Vec<T> {
     vec.push(item);
     vec

apollo-federation/src/connectors/json_selection/methods.rs

@@ -48,6 +48,7 @@ pub(super) enum ArrowMethod {
     Gt,
     Lt,
     Not,
+    ToString,
 
     // Future methods:
     TypeOf,
@@ -167,6 +168,7 @@ impl std::ops::Deref for ArrowMethod {
             Self::Gt => &public::GtMethod,
             Self::Lt => &public::LtMethod,
             Self::Not => &public::NotMethod,
+            Self::ToString => &public::ToStringMethod,
 
             // Future methods:
             Self::TypeOf => &future::TypeOfMethod,
@@ -224,6 +226,7 @@ impl ArrowMethod {
             "ne" => Some(Self::Ne),
             "gt" => Some(Self::Gt),
             "lt" => Some(Self::Lt),
+            "toString" => Some(Self::ToString),
             _ => None,
         };
 
@@ -259,6 +262,7 @@ impl ArrowMethod {
                 | Self::Gt
                 | Self::Lt
                 | Self::Not
+                | Self::ToString
         )
     }
 }

apollo-federation/src/connectors/json_selection/methods/public/join_not_null.rs

@@ -8,6 +8,7 @@ use crate::connectors::json_selection::ApplyToError;
 use crate::connectors::json_selection::ApplyToInternal;
 use crate::connectors::json_selection::MethodArgs;
 use crate::connectors::json_selection::VarsWithPathsMap;
+use crate::connectors::json_selection::helpers::json_to_string;
 use crate::connectors::json_selection::immutable::InputPath;
 use crate::connectors::json_selection::location::Ranged;
 use crate::connectors::json_selection::location::WithRange;
@@ -72,16 +73,9 @@ fn join_not_null_method(
     };
 
     fn to_string(value: &JSON, method_name: &str) -> Result<Option<String>, String> {
-        match value {
-            JSON::Bool(b) => Ok(Some(b.then_some("true").unwrap_or("false").to_string())),
-            JSON::Number(number) => Ok(Some(number.to_string())),
-            JSON::String(byte_string) => Ok(Some(byte_string.as_str().to_string())),
-            JSON::Null => Ok(None),
-            JSON::Array(_) | JSON::Object(_) => Err(format!(
-                "Method ->{} requires an array of scalar values as input",
-                method_name
-            )),
-        }
+        json_to_string(value).map_err(|_| {
+            format!("Method ->{method_name} requires an array of scalar values as input",)
+        })
     }
 
     let joined = match data {

apollo-federation/src/connectors/json_selection/methods/public/mod.rs

@@ -39,3 +39,5 @@ mod lt;
 pub(crate) use lt::LtMethod;
 mod not;
 pub(crate) use not::NotMethod;
+mod to_string;
+pub(crate) use to_string::ToStringMethod;