fix: remove silent guard in InsertOperator for null/empty documents (#7)

Author: dieppa

ANALYSIS.md

@@ -33,10 +33,10 @@ YAML -> MongoOperation (deserialized) -> MongoOperationValidator -> MongoOperati
 **Original issue:** `MongoChangeTemplate.rollback()` did not call `MongoOperationValidator.validate()` before executing, so malformed rollback YAML ran unchecked.
 **Resolution:** Structural validation was moved into `MongoOperation.validate()` (implementing the `TemplatePayload` interface). The Flamingock framework now calls `validate()` on both apply and rollback payloads at load time via `AbstractTemplateLoadedChange.getValidationErrors()`, which invokes `validateApplyPayload()` and `validateRollbackPayload()` before any change executes. The separate `MongoOperationValidator`, `ValidationError`, and `MongoTemplateValidationException` classes were deleted as part of this refactoring. Validation is no longer a responsibility of the template's `apply()`/`rollback()` methods — the framework handles it uniformly for all payloads.
 
-### #2 - HIGH: InsertOperator silently swallows null/empty documents, bypassing validation
-**File:** `InsertOperator.java:37-39`
-**Impact:** The validator correctly rejects empty/null documents (`MongoOperationValidator.java:268-271`), but `InsertOperator.applyInternal()` has a redundant guard: `if(op.getDocuments() == null || op.getDocuments().isEmpty()) { return; }`. If the validator is ever bypassed (e.g., rollback path per issue #1, or direct operator usage), the insert silently does nothing. A migration that is supposed to seed data will silently succeed without inserting anything - an invisible data loss scenario.
-**Fix:** Remove the silent return. Let the operator trust the validator. If documents are null/empty at this point, it's a bug that should throw, not be silenced.
+### #2 - ~~HIGH: InsertOperator silently swallows null/empty documents, bypassing validation~~ RESOLVED
+**Status:** Fixed by removing the silent guard.
+**Original issue:** `InsertOperator.applyInternal()` had a redundant guard `if(op.getDocuments() == null || op.getDocuments().isEmpty()) { return; }` that silently did nothing instead of failing when documents were missing.
+**Resolution:** The silent guard was removed from `InsertOperator.applyInternal()`. Structural validation now runs at load time via `InsertParametersValidator` (called from `MongoOperation.validate()`), which catches null, empty, and malformed documents before any change executes. The operator no longer needs a defensive check — if documents are invalid, the framework rejects the change at load time.
 
 ### #3 - HIGH: `modifyCollection` has ZERO parameter validation
 **File:** `MongoOperationValidator.java:239-240`

src/main/java/io/flamingock/template/mongodb/MongoChangeTemplate.java

@@ -81,12 +81,26 @@ public MongoChangeTemplate() {
         super(MongoOperation.class);
     }
 
+    /**
+     * Executes the apply operation for the current step.
+     * <p>
+     * Precondition: {@code applyPayload} has been structurally validated at load time
+     * via {@link MongoOperation#validate()}, which delegates to the appropriate
+     * {@code ParametersValidator} for each operation type. No validation is performed here.
+     */
     @Apply
     public void apply(MongoDatabase db, @Nullable ClientSession clientSession) {
         validateSession(clientSession);
         applyPayload.getOperator(db).apply(clientSession);
     }
 
+    /**
+     * Executes the rollback operation for the current step.
+     * <p>
+     * Precondition: {@code rollbackPayload} has been structurally validated at load time
+     * via {@link MongoOperation#validate()}, which delegates to the appropriate
+     * {@code ParametersValidator} for each operation type. No validation is performed here.
+     */
     @Rollback
     public void rollback(MongoDatabase db, @Nullable ClientSession clientSession) {
         validateSession(clientSession);

src/main/java/io/flamingock/template/mongodb/model/operator/InsertOperator.java

@@ -31,12 +31,15 @@ public InsertOperator(MongoDatabase mongoDatabase, MongoOperation operation) {
         super(mongoDatabase, operation, true);
     }
 
+    /**
+     * Executes the insert operation against MongoDB.
+     * <p>
+     * Precondition: {@code op.getDocuments()} is guaranteed non-null and non-empty
+     * by {@code InsertParametersValidator} at load time, so no defensive check is needed here.
+     */
     @Override
     protected void applyInternal(ClientSession clientSession) {
         MongoCollection<Document> collection = mongoDatabase.getCollection(op.getCollection());
-        if(op.getDocuments() == null || op.getDocuments().isEmpty()) {
-            return;
-        }
 
         if(op.getDocuments().size() == 1) {
             insertOne(clientSession, collection);

src/test/java/io/flamingock/template/mongodb/operations/InsertOperatorTest.java

@@ -23,6 +23,7 @@
 import io.flamingock.template.mongodb.model.MongoOperation;
 import io.flamingock.template.mongodb.model.operator.InsertOperator;
 import org.bson.Document;
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.DisplayName;
@@ -144,10 +145,8 @@ void insertManyDocumentsTest() {
     }
 
     @Test
-    @DisplayName("WHEN insert operator is applied with empty documents THEN nothing is inserted")
+    @DisplayName("WHEN insert operator is applied with empty documents THEN exception is thrown")
     void insertEmptyDocumentsTest() {
-        assertEquals(0, getDocumentCount(), "Collection should be empty before insert");
-
         MongoOperation operation = new MongoOperation();
         operation.setType("insert");
         operation.setCollection(COLLECTION_NAME);
@@ -157,9 +156,9 @@ void insertEmptyDocumentsTest() {
         operation.setParameters(params);
 
         InsertOperator operator = new InsertOperator(mongoDatabase, operation);
-        operator.apply(null);
-
-        assertEquals(0, getDocumentCount(), "Collection should still be empty");
+        // Empty documents should be caught by InsertParametersValidator at load time.
+        // If the operator is called directly with empty documents, the MongoDB driver throws.
+        Assertions.assertThrows(IllegalArgumentException.class, () -> operator.apply(null));
     }
 
     private long getDocumentCount() {