fix: createRecipe checks name is unique

Uses JPA Repository.exists(Example<T>); Changed no-arg constuctor of
Recipe to public.

commons/src/main/java/commons/Recipe.java

@@ -83,7 +83,7 @@ public class Recipe {
     private List<String> preparationSteps = new ArrayList<>();
 
     @SuppressWarnings("unused")
-    private Recipe() {
+    public Recipe() {
         // for object mapper
     }
 

server/src/main/java/server/api/RecipeController.java

@@ -2,6 +2,7 @@ package server.api;
 
 import commons.Recipe;
 
+import org.springframework.data.domain.Example;
 import org.springframework.data.domain.PageRequest;
 import org.springframework.http.ResponseEntity;
 
@@ -86,11 +87,20 @@ public class RecipeController {
      *     Inserts a new recipe into the repository
      * </p>
      * @param recipe The new recipe as a request body
-     * @return 200 OK with the recipe you added; or 400 Bad Request if the recipe already exists
+     * @return 200 OK with the recipe you added; or 400 Bad Request if the recipe already exists by name
      */
     @PutMapping("/recipe/new")
     public ResponseEntity<Recipe> createRecipe(@RequestBody Recipe recipe) {
-        if (recipeRepository.existsById(recipe.getId())) {
+
+        // We initialize a new example recipe with the name of input recipe
+        // This is the only attribute we are concerned about making sure it's unique
+        Recipe example = new Recipe();
+        example.setName(recipe.getName());
+
+        /*  Here we use very funny JPA magic repository.exists(Example<Recipe>)
+            We check if any recipe in the repository has the same name as the input
+         */
+        if (recipeRepository.exists(Example.of(example))) {
             return ResponseEntity.badRequest().build();
         }