Index: Application/Ringtoets/src/Application.Ringtoets.Storage/Create/HydraulicBoundaryLocationCalculationCreateExtensions.cs
===================================================================
diff -u -r87b871f1ce03d1909e2402890a01154c5d9b797b -r06b82f3137d117242ecf5f69bded51c103216c0a
--- Application/Ringtoets/src/Application.Ringtoets.Storage/Create/HydraulicBoundaryLocationCalculationCreateExtensions.cs	(.../HydraulicBoundaryLocationCalculationCreateExtensions.cs)	(revision 87b871f1ce03d1909e2402890a01154c5d9b797b)
+++ Application/Ringtoets/src/Application.Ringtoets.Storage/Create/HydraulicBoundaryLocationCalculationCreateExtensions.cs	(.../HydraulicBoundaryLocationCalculationCreateExtensions.cs)	(revision 06b82f3137d117242ecf5f69bded51c103216c0a)
@@ -36,19 +36,26 @@
         /// <see cref="HydraulicBoundaryLocationCalculation"/>.
         /// </summary>
         /// <param name="calculation">The calculation to create a database entity for.</param>
+        /// <param name="registry">The object keeping track of create operations.</param>
         /// <returns>A new <see cref="HydraulicLocationCalculationEntity"/>.</returns>
-        /// <exception cref="ArgumentNullException">Thrown when <paramref name="calculation"/>
-        /// is <c>null</c>.</exception>
-        internal static HydraulicLocationCalculationEntity Create(this HydraulicBoundaryLocationCalculation calculation)
+        /// <exception cref="ArgumentNullException">Thrown when any parameter is <c>null</c>.</exception>
+        public static HydraulicLocationCalculationEntity Create(this HydraulicBoundaryLocationCalculation calculation,
+                                                                PersistenceRegistry registry)
         {
             if (calculation == null)
             {
                 throw new ArgumentNullException(nameof(calculation));
             }
 
+            if (registry == null)
+            {
+                throw new ArgumentNullException(nameof(registry));
+            }
+
             var hydraulicLocationCalculationEntity = new HydraulicLocationCalculationEntity
             {
-                ShouldIllustrationPointsBeCalculated = Convert.ToByte(calculation.InputParameters.ShouldIllustrationPointsBeCalculated)
+                ShouldIllustrationPointsBeCalculated = Convert.ToByte(calculation.InputParameters.ShouldIllustrationPointsBeCalculated),
+                HydraulicLocationEntity = registry.Get<HydraulicLocationEntity>(calculation.HydraulicBoundaryLocation)
             };
 
             CreateHydraulicBoundaryLocationOutput(hydraulicLocationCalculationEntity, calculation.Output);
Index: Application/Ringtoets/test/Application.Ringtoets.Storage.Test/Create/HydraulicBoundaryLocationCalculationCreateExtensionsTest.cs
===================================================================
diff -u -r8aca9e7a0c1324719d2b609a05df4739e6c095a9 -r06b82f3137d117242ecf5f69bded51c103216c0a
--- Application/Ringtoets/test/Application.Ringtoets.Storage.Test/Create/HydraulicBoundaryLocationCalculationCreateExtensionsTest.cs	(.../HydraulicBoundaryLocationCalculationCreateExtensionsTest.cs)	(revision 8aca9e7a0c1324719d2b609a05df4739e6c095a9)
+++ Application/Ringtoets/test/Application.Ringtoets.Storage.Test/Create/HydraulicBoundaryLocationCalculationCreateExtensionsTest.cs	(.../HydraulicBoundaryLocationCalculationCreateExtensionsTest.cs)	(revision 06b82f3137d117242ecf5f69bded51c103216c0a)
@@ -34,10 +34,24 @@
     public class HydraulicBoundaryLocationCalculationCreateExtensionsTest
     {
         [Test]
+        public void Create_RegistryNull_ThrowsArgumentNullException()
+        {
+            // Setup
+            var calculation = new HydraulicBoundaryLocationCalculation(new TestHydraulicBoundaryLocation());
+
+            // Call
+            TestDelegate call = () => calculation.Create(null);
+
+            // Assert
+            var exception = Assert.Throws<ArgumentNullException>(call);
+            Assert.AreEqual("registry", exception.ParamName);
+        }
+
+        [Test]
         public void Create_CalculationNull_ThrowsArgumentNullException()
         {
             // Call
-            TestDelegate call = () => ((HydraulicBoundaryLocationCalculation) null).Create();
+            TestDelegate call = () => ((HydraulicBoundaryLocationCalculation) null).Create(new PersistenceRegistry());
 
             // Assert
             var exception = Assert.Throws<ArgumentNullException>(call);
@@ -50,7 +64,9 @@
             // Setup
             var random = new Random(33);
             bool shouldIllustrationPointsBeCalculated = random.NextBoolean();
-            var calculation = new HydraulicBoundaryLocationCalculation(new TestHydraulicBoundaryLocation())
+
+            var hydraulicBoundaryLocation = new TestHydraulicBoundaryLocation();
+            var calculation = new HydraulicBoundaryLocationCalculation(hydraulicBoundaryLocation)
             {
                 InputParameters =
                 {
@@ -59,8 +75,12 @@
                 Output = null
             };
 
+            var registry = new PersistenceRegistry();
+            var hydraulicLocationEntity = new HydraulicLocationEntity();
+            registry.Register(hydraulicLocationEntity, hydraulicBoundaryLocation);
+
             // Call
-            HydraulicLocationCalculationEntity entity = calculation.Create();
+            HydraulicLocationCalculationEntity entity = calculation.Create(registry);
 
             // Assert
             Assert.IsNotNull(entity);
@@ -74,7 +94,9 @@
             // Setup
             var random = new Random(33);
             bool shouldIllustrationPointsBeCalculated = random.NextBoolean();
-            var calculation = new HydraulicBoundaryLocationCalculation(new TestHydraulicBoundaryLocation())
+
+            var hydraulicBoundaryLocation = new TestHydraulicBoundaryLocation();
+            var calculation = new HydraulicBoundaryLocationCalculation(hydraulicBoundaryLocation)
             {
                 InputParameters =
                 {
@@ -86,8 +108,12 @@
                     null)
             };
 
+            var registry = new PersistenceRegistry();
+            var hydraulicLocationEntity = new HydraulicLocationEntity();
+            registry.Register(hydraulicLocationEntity, hydraulicBoundaryLocation);
+
             // Call
-            HydraulicLocationCalculationEntity entity = calculation.Create();
+            HydraulicLocationCalculationEntity entity = calculation.Create(registry);
 
             // Assert
             Assert.IsNotNull(entity);
@@ -103,5 +129,25 @@
             Assert.IsNull(outputEntity.GeneralResultSubMechanismIllustrationPointEntity);
             Assert.AreEqual(Convert.ToByte(expectedOutput.CalculationConvergence), outputEntity.CalculationConvergence);
         }
+
+        [Test]
+        public void Create_CalculationWithAlreadyCreatedHydraulicBoundaryLocation_ReturnsEntityWithHydraulicBoundaryLocationEntity()
+        {
+            // Setup
+            var hydraulicLocation = new TestHydraulicBoundaryLocation();
+
+            var registry = new PersistenceRegistry();
+            var hydraulicLocationEntity = new HydraulicLocationEntity();
+            registry.Register(hydraulicLocationEntity, hydraulicLocation);
+
+            var calculation = new HydraulicBoundaryLocationCalculation(hydraulicLocation);
+            
+            // Call
+            HydraulicLocationCalculationEntity entity = calculation.Create(registry);
+
+            // Assert
+            Assert.IsNotNull(entity);
+            Assert.AreSame(hydraulicLocationEntity, entity.HydraulicLocationEntity);
+        }
     }
 }
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