Do not generate FK on non-generated unique constraint

src/NHibernate.Test/DialectTest/SchemaTests/DialectNotSupportingNullInUnique.cs

@@ -29,6 +29,7 @@ protected HbmMapping GetMappings()
 							m.UniqueKey("Test");
 						});
 					rc.Property(x => x.Name2, m => m.UniqueKey("Test"));
+					rc.ManyToOne(e => e.Parent, m => { m.PropertyRef("Name"); });
 				});
 
 			return mapper.CompileMappingForAllExplicitlyAddedEntities();
@@ -42,7 +43,8 @@ public void ScriptGenerationForDialectNotSupportingNullInUnique()
 
 			var script = configuration.GenerateSchemaCreationScript(new DialectNotSupportingNullInUnique());
 
-			Assert.That(script, Has.None.Contains("unique"));
+			Assert.That(script, Has.None.Match(@".*\bunique\b.*").IgnoreCase);
+			Assert.That(script, Has.None.Match(@".*\bforeign key\b.*").IgnoreCase);
 		}
 	}
 }

src/NHibernate.Test/DialectTest/SchemaTests/Entity.cs

@@ -8,5 +8,6 @@ public class Entity
 		public virtual string Name { get; set; }
 		public virtual string Name1 { get; set; }
 		public virtual string Name2 { get; set; }
+		public virtual Entity Parent { get; set; }
 	}
 }

src/NHibernate/Cfg/Configuration.cs

@@ -851,7 +851,7 @@ public string[] GenerateDropSchemaScript(Dialect.Dialect dialect)
 					{
 						foreach (var fk in table.ForeignKeyIterator)
 						{
-							if (fk.HasPhysicalConstraint && IncludeAction(fk.ReferencedTable.SchemaActions, SchemaAction.Drop))
+							if (fk.IsGenerated(dialect) && IncludeAction(fk.ReferencedTable.SchemaActions, SchemaAction.Drop))
 							{
 								script.Add(fk.SqlDropString(dialect, defaultCatalog, defaultSchema));
 							}
@@ -939,7 +939,7 @@ public string[] GenerateSchemaCreationScript(Dialect.Dialect dialect)
 					{
 						foreach (var fk in table.ForeignKeyIterator)
 						{
-							if (fk.HasPhysicalConstraint && IncludeAction(fk.ReferencedTable.SchemaActions, SchemaAction.Export))
+							if (fk.IsGenerated(dialect) && IncludeAction(fk.ReferencedTable.SchemaActions, SchemaAction.Export))
 							{
 								script.Add(fk.SqlCreateString(dialect, mapping, defaultCatalog, defaultSchema));
 							}
@@ -2344,7 +2344,7 @@ public string[] GenerateSchemaUpdateScript(Dialect.Dialect dialect, IDatabaseMet
 					{
 						foreach (var fk in table.ForeignKeyIterator)
 						{
-							if (fk.HasPhysicalConstraint && IncludeAction(fk.ReferencedTable.SchemaActions, SchemaAction.Update))
+							if (fk.IsGenerated(dialect) && IncludeAction(fk.ReferencedTable.SchemaActions, SchemaAction.Update))
 							{
 								bool create = tableInfo == null
 											  ||

src/NHibernate/Mapping/ForeignKey.cs

@@ -234,5 +234,20 @@ public bool IsReferenceToPrimaryKey
 		}
 
 		public string GeneratedConstraintNamePrefix => "FK_";
+
+		public override bool IsGenerated(Dialect.Dialect dialect)
+		{
+			if (!HasPhysicalConstraint)
+				return false;
+			if (dialect.SupportsNullInUnique || IsReferenceToPrimaryKey)
+				return true;
+
+			foreach (var column in ReferencedColumns)
+			{
+				if (column.IsNullable)
+					return false;
+			}
+			return true;
+		}
 	}
 }

src/NHibernate/Mapping/Table.cs

@@ -453,7 +453,7 @@ public string SqlCreateString(Dialect.Dialect dialect, IMapping p, string defaul
 			{
 				foreach (ForeignKey foreignKey in ForeignKeyIterator)
 				{
-					if (foreignKey.HasPhysicalConstraint)
+					if (foreignKey.IsGenerated(dialect))
 					{
 						buf.Append(",").Append(foreignKey.SqlConstraintString(dialect, foreignKey.Name, defaultCatalog, defaultSchema));
 					}