[JSONSelection] Support conditional fragment selections.

apollo-federation/src/sources/connect/json_selection/README.md

@@ -83,11 +83,18 @@ below.
 ```ebnf
 JSONSelection        ::= PathSelection | NamedSelection*
 SubSelection         ::= "{" NamedSelection* "}"
-NamedSelection       ::= NamedPathSelection | PathWithSubSelection | NamedFieldSelection | NamedGroupSelection
+NamedSelection       ::= NamedPathSelection
+                       | PathWithSubSelection
+                       | NamedFieldSelection
+                       | NamedGroupSelection
+                       | ConditionalSelection
 NamedPathSelection   ::= Alias PathSelection
 NamedFieldSelection  ::= Alias? Key SubSelection?
 NamedGroupSelection  ::= Alias SubSelection
 Alias                ::= Key ":"
+ConditionalSelection ::= "..." ConditionalTest
+ConditionalTest      ::= "if" "(" Path ")" SubSelection ConditionalElse?
+ConditionalElse      ::= "else" (ConditionalTest | SubSelection)
 Path                 ::= VarPath | KeyPath | AtPath | ExprPath
 PathSelection        ::= Path SubSelection?
 PathWithSubSelection ::= Path SubSelection
@@ -351,6 +358,37 @@ from the input JSON to match the desired output shape.
 In addition to renaming, `Alias` can provide names to otherwise anonymous
 structures, such as those selected by `PathSelection` or `NamedGroupSelection`.
 
+### `ConditionalSelection ::=`
+
+![ConditionalSelection](./grammar/ConditionalSelection.svg)
+
+The `...` token signifies the beginning of a `ConditionalSelection` element,
+which is a kind of named selection that may appear multiple times within any
+`SubSelection`.
+
+The `...` is always followed by a `ConditionalTest`, since unconditional spreads
+are rarely useful in this language, and can almost always be rewritten by
+unwrapping the fields and removing the `...`.
+
+### `ConditionalTest ::=`
+
+![ConditionalTest](./grammar/ConditionalTest.svg)
+
+`ConditionalTest` uses the `if` keyword followed by a parenthesized `Path` that
+should evaluate to a boolean value. The `Path` is used to determine whether the
+`SubSelection` or `ConditionalElse` should be selected.
+
+### `ConditionalElse ::=`
+
+![ConditionalElse](./grammar/ConditionalElse.svg)
+
+`ConditionalElse` is an optional trailing clause of `ConditionalTest`, which
+allows for typical `if`/`else`-style boolean control flow.
+
+Note that `ConditionalElse` may expand to an `else` keyword followed by a
+`ConditionalTest`, so the `ConditionalTest` and `ConditionalElse` rules are
+mutually recursive.
+
 ### `Path ::=`
 
 ![Path](./grammar/Path.svg)

apollo-federation/src/sources/connect/json_selection/apply_to.rs

@@ -256,6 +256,7 @@ impl ApplyToInternal for NamedSelection {
                     ));
                 }
             }
+
             Self::Path(alias_opt, path_selection) => {
                 let (value_opt, apply_errors) =
                     path_selection.apply_to_path(data, vars, input_path);
@@ -308,19 +309,67 @@ impl ApplyToInternal for NamedSelection {
                     ));
                 }
             }
+
             Self::Group(alias, sub_selection) => {
                 let (value_opt, apply_errors) = sub_selection.apply_to_path(data, vars, input_path);
                 errors.extend(apply_errors);
                 if let Some(value) = value_opt {
                     output.insert(alias.name(), value);
                 }
             }
+
+            Self::Spread(spread) => {
+                let (spread_opt, spread_errors) = spread.apply_to_path(data, vars, input_path);
+                errors.extend(spread_errors);
+                if let Some(JSON::Object(spread)) = spread_opt {
+                    // TODO Better merge strategy for conflicting fields
+                    output.extend(spread);
+                }
+            }
         };
 
         (Some(JSON::Object(output)), errors)
     }
 }
 
+impl ApplyToInternal for ConditionalTest {
+    fn apply_to_path(
+        &self,
+        data: &JSON,
+        vars: &VarsWithPathsMap,
+        input_path: &InputPath<JSON>,
+    ) -> (Option<JSON>, Vec<ApplyToError>) {
+        let (test_value, test_errors) = self.test.apply_to_path(data, vars, input_path);
+        if let Some(JSON::Bool(true)) = test_value {
+            self.when_true
+                .apply_to_path(data, vars, input_path)
+                .prepend_errors(test_errors)
+        } else if let Some(when_else) = &self.when_else {
+            when_else
+                .apply_to_path(data, vars, input_path)
+                .prepend_errors(test_errors)
+        } else {
+            (None, test_errors)
+        }
+    }
+}
+
+impl ApplyToInternal for ConditionalElse {
+    fn apply_to_path(
+        &self,
+        data: &JSON,
+        vars: &VarsWithPathsMap,
+        input_path: &InputPath<JSON>,
+    ) -> (Option<JSON>, Vec<ApplyToError>) {
+        match self {
+            Self::Else(selection) => selection.apply_to_path(data, vars, input_path),
+            Self::ElseIf(conditional_test) => {
+                conditional_test.apply_to_path(data, vars, input_path)
+            }
+        }
+    }
+}
+
 impl ApplyToInternal for PathSelection {
     fn apply_to_path(
         &self,
@@ -2055,4 +2104,119 @@ mod tests {
             (Some(json!(123)), vec![],),
         );
     }
+
+    #[test]
+    fn test_conditional_fragments() {
+        let book = json!({
+            "kind": "book",
+            "title": "The Great Gatsby",
+            "author": "F. Scott Fitzgerald",
+        });
+
+        let movie = json!({
+            "kind": "movie",
+            "title": "The Great Gatsby",
+            "director": "Baz Luhrmann",
+        });
+
+        let product = json!({
+            "kind": "product",
+            "title": "Jay Gatsby Action Figure",
+            "price": 19.99,
+        });
+
+        let selection = selection!(
+            r#"
+            title
+            ... if (kind->eq("book")) {
+                author
+            } else if (kind->eq("movie")) {
+                director
+            } else {
+                kind
+            }
+            "#
+        );
+
+        assert_eq!(
+            selection.apply_to(&book),
+            (
+                Some(json!({
+                    "title": "The Great Gatsby",
+                    "author": "F. Scott Fitzgerald"
+                })),
+                vec![]
+            )
+        );
+        assert_eq!(
+            selection.apply_to(&movie),
+            (
+                Some(json!({
+                    "title": "The Great Gatsby",
+                    "director": "Baz Luhrmann"
+                })),
+                vec![]
+            )
+        );
+        assert_eq!(
+            selection.apply_to(&product),
+            (
+                Some(json!({
+                    "title": "Jay Gatsby Action Figure",
+                    "kind": "product"
+                })),
+                vec![]
+            )
+        );
+
+        let selection_with_typenames = selection!(
+            r#"
+            title
+            ... if (kind->eq("book")) {
+                __typename: $("Book")
+                author
+            } else if (kind->eq("movie")) {
+                __typename: $("Movie")
+                director
+            } else {
+                __typename: $("Product")
+                kind
+            }
+            "#
+        );
+
+        assert_eq!(
+            selection_with_typenames.apply_to(&book),
+            (
+                Some(json!({
+                    "title": "The Great Gatsby",
+                    "__typename": "Book",
+                    "author": "F. Scott Fitzgerald"
+                })),
+                vec![]
+            )
+        );
+        assert_eq!(
+            selection_with_typenames.apply_to(&movie),
+            (
+                Some(json!({
+                    "title": "The Great Gatsby",
+                    "__typename": "Movie",
+                    "director": "Baz Luhrmann"
+                })),
+                vec![]
+            )
+        );
+        assert_eq!(
+            selection_with_typenames.apply_to(&product),
+            (
+                Some(json!({
+                    "title": "Jay Gatsby Action Figure",
+                    "__typename": "Product",
+                    "kind": "product"
+                })),
+                vec![]
+            )
+        );
+    }
 }