diff --git a/src/Digitall.Dataverse.Testing/OrganizationRequests/DeepInsertProcessor.cs b/src/Digitall.Dataverse.Testing/OrganizationRequests/DeepInsertProcessor.cs
index 6be3dbf..6f8b99c 100644
--- a/src/Digitall.Dataverse.Testing/OrganizationRequests/DeepInsertProcessor.cs
+++ b/src/Digitall.Dataverse.Testing/OrganizationRequests/DeepInsertProcessor.cs
@@ -68,21 +68,55 @@ private static void ProcessOneToMany(
         EntityCollection children,
         FakeOrganizationService state)
     {
-        if (metadata.ReferencedEntity != parentLogicalName)
+        if (metadata.ReferencedEntity == parentLogicalName)
         {
-            ErrorFactory.ThrowFault(
-                ErrorCodes.InvalidArgument,
-                $"Relationship '{metadata.SchemaName}' references entity '{metadata.ReferencedEntity}' " +
-                $"but the parent entity is '{parentLogicalName}'. The relationship metadata does not match the parent.");
+            // Parent is the "one" side → set FK on each child pointing to parent
+            var parentRef = new EntityReference(parentLogicalName, parentId);
+
+            foreach (var child in children.Entities)
+            {
+                child[metadata.ReferencingAttribute] = parentRef;
+                state.Create(child);
+            }
         }
+        else if (metadata.ReferencingEntity == parentLogicalName)
+        {
+            // Parent is the "many" side (N:1) → lookup can only point to a single record
+            if (children.Entities.Count > 1)
+            {
+                ErrorFactory.ThrowFault(
+                    ErrorCodes.InvalidArgument,
+                    $"Relationship '{metadata.SchemaName}' is used from the referencing (N:1) side. " +
+                    "A lookup attribute can only reference a single entity, but multiple related entities were provided.");
+            }
 
-        var parentRef = new EntityReference(parentLogicalName, parentId);
+            var child = children.Entities[0];
 
-        foreach (var child in children.Entities)
+            if (child.LogicalName != metadata.ReferencedEntity)
+            {
+                ErrorFactory.ThrowFault(
+                    ErrorCodes.InvalidArgument,
+                    $"Relationship '{metadata.SchemaName}' expects referenced entity '{metadata.ReferencedEntity}' " +
+                    $"but the related entity is '{child.LogicalName}'.");
+            }
+
+            // Create child first, then set FK on parent pointing to child
+            var childId = state.Create(child);
+            var childRef = new EntityReference(metadata.ReferencedEntity, childId);
+
+            var parentUpdate = new Entity(parentLogicalName, parentId)
+            {
+                [metadata.ReferencingAttribute] = childRef
+            };
+            state.Update(parentUpdate);
+        }
+        else
         {
-            // Set the foreign key (lookup) on the child pointing to the parent
-            child[metadata.ReferencingAttribute] = parentRef;
-            state.Create(child);
+            ErrorFactory.ThrowFault(
+                ErrorCodes.InvalidArgument,
+                $"Relationship '{metadata.SchemaName}' is between '{metadata.ReferencedEntity}' and " +
+                $"'{metadata.ReferencingEntity}' but the parent entity is '{parentLogicalName}'. " +
+                "The relationship metadata does not match the parent.");
         }
     }
 
diff --git a/tests/Digitall.Dataverse.Testing.Tests/OrganizationRequests/CreateFakeDeepInsertTests.cs b/tests/Digitall.Dataverse.Testing.Tests/OrganizationRequests/CreateFakeDeepInsertTests.cs
index 23fd4bd..3534d16 100644
--- a/tests/Digitall.Dataverse.Testing.Tests/OrganizationRequests/CreateFakeDeepInsertTests.cs
+++ b/tests/Digitall.Dataverse.Testing.Tests/OrganizationRequests/CreateFakeDeepInsertTests.cs
@@ -5,6 +5,7 @@
 using Microsoft.Xrm.Sdk;
 using Microsoft.Xrm.Sdk.Messages;
 using Microsoft.Xrm.Sdk.Metadata;
+using Microsoft.Xrm.Sdk.Query;
 
 namespace Digitall.Dataverse.Testing.Tests.OrganizationRequests;
 
@@ -216,4 +217,107 @@ account.RelatedEntities[new Relationship("systemuserroles_association")] =
 
         await Assert.That(act).Throws<FaultException>();
     }
+
+    [Test]
+    public async Task Create_WithReversedOneToMany_SetsForeignKeyOnParent()
+    {
+        // Scenario: calendarrule (Referencing) has a lookup to calendar (Referenced)
+        // Deep insert from calendarrule side should create the calendar and set FK on calendarrule
+        _sut.State.Relationships["calendarrule_innercalendar"] = new OneToManyRelationshipMetadata
+        {
+            SchemaName = "calendarrule_innercalendar",
+            ReferencedEntity = "calendar",
+            ReferencedAttribute = "calendarid",
+            ReferencingEntity = "calendarrule",
+            ReferencingAttribute = "innercalendarid"
+        };
+
+        // Also register the parent relationship: calendar → calendarrule
+        _sut.State.Relationships["calendar_calendar_rules"] = new OneToManyRelationshipMetadata
+        {
+            SchemaName = "calendar_calendar_rules",
+            ReferencedEntity = "calendar",
+            ReferencedAttribute = "calendarid",
+            ReferencingEntity = "calendarrule",
+            ReferencingAttribute = "calendarid"
+        };
+
+        var calendar = new Entity("calendar") { Id = Guid.NewGuid(), ["name"] = "Business Hours" };
+        var innerCalendar = new Entity("calendar") { ["name"] = "Inner Schedule" };
+        var calendarRule = new Entity("calendarrule") { ["description"] = "Rule 1" };
+
+        // Nested deep insert: calendar → calendarrule → innercalendar (reverse direction)
+        calendarRule.RelatedEntities[new Relationship("calendarrule_innercalendar")] =
+            new EntityCollection([innerCalendar]);
+
+        calendar.RelatedEntities[new Relationship("calendar_calendar_rules")] =
+            new EntityCollection([calendarRule]);
+
+        _sut.Execute(new CreateRequest { Target = calendar });
+
+        // Verify all entities were created
+        var calendars = _sut.CreateQuery("calendar").ToList();
+        var rules = _sut.CreateQuery("calendarrule").ToList();
+
+        await Assert.That(calendars).Count().IsEqualTo(2); // parent + inner
+        await Assert.That(rules).Count().IsEqualTo(1);
+
+        // Verify the FK on calendarrule points to the inner calendar
+        var createdRule = rules.Single();
+        var innerCalRef = createdRule.GetAttributeValue<EntityReference>("innercalendarid");
+        await Assert.That(innerCalRef).IsNotNull();
+        await Assert.That(innerCalRef!.LogicalName).IsEqualTo("calendar");
+
+        // The inner calendar should be the one named "Inner Schedule"
+        var innerCal = _sut.Retrieve("calendar", innerCalRef.Id, new ColumnSet(true));
+        await Assert.That(innerCal.GetAttributeValue<string>("name")).IsEqualTo("Inner Schedule");
+    }
+
+    [Test]
+    public async Task Create_WithReversedOneToMany_MultipleChildren_ThrowsFault()
+    {
+        // N:1 direction: a lookup can only point to one record
+        _sut.State.Relationships["calendarrule_innercalendar"] = new OneToManyRelationshipMetadata
+        {
+            SchemaName = "calendarrule_innercalendar",
+            ReferencedEntity = "calendar",
+            ReferencedAttribute = "calendarid",
+            ReferencingEntity = "calendarrule",
+            ReferencingAttribute = "innercalendarid"
+        };
+
+        var rule = new Entity("calendarrule") { Id = Guid.NewGuid(), ["description"] = "Rule" };
+        var cal1 = new Entity("calendar") { ["name"] = "Cal 1" };
+        var cal2 = new Entity("calendar") { ["name"] = "Cal 2" };
+
+        rule.RelatedEntities[new Relationship("calendarrule_innercalendar")] =
+            new EntityCollection([cal1, cal2]); // Two children on N:1 → invalid
+
+        var act = () => _sut.Execute(new CreateRequest { Target = rule });
+
+        await Assert.That(act).Throws<FaultException>();
+    }
+
+    [Test]
+    public async Task Create_WithReversedOneToMany_WrongChildLogicalName_ThrowsFault()
+    {
+        _sut.State.Relationships["calendarrule_innercalendar"] = new OneToManyRelationshipMetadata
+        {
+            SchemaName = "calendarrule_innercalendar",
+            ReferencedEntity = "calendar",
+            ReferencedAttribute = "calendarid",
+            ReferencingEntity = "calendarrule",
+            ReferencingAttribute = "innercalendarid"
+        };
+
+        var rule = new Entity("calendarrule") { Id = Guid.NewGuid(), ["description"] = "Rule" };
+        var wrongEntity = new Entity("account") { ["name"] = "Not a calendar" };
+
+        rule.RelatedEntities[new Relationship("calendarrule_innercalendar")] =
+            new EntityCollection([wrongEntity]);
+
+        var act = () => _sut.Execute(new CreateRequest { Target = rule });
+
+        await Assert.That(act).Throws<FaultException>();
+    }
 }