Fix NRE in SchemaUpdate and SchemaExport Execute method for collection mappings (#2429)

Author: maca88

src/NHibernate.Test/Tools/hbm2ddl/SchemaMetadataUpdaterTest/HeavyEntity.hbm.xml

@@ -4,16 +4,27 @@
 				   assembly="NHibernate.Test"
 				   default-lazy="false">
 
-	<class name="Order">
-		<id type="int">
-			<generator class="native"/>
-		</id>
-		<property name="Select"/>
-		<property name="From"/>
-		<property name="And"/>
-		<property name="Column"/>
-		<property name="Name"/>
-		<property name="Abracadabra"/>
-	</class>
+  <class name="Order">
+    <id type="int">
+      <generator class="native"/>
+    </id>
+    <property name="Select"/>
+    <property name="From"/>
+    <property name="And"/>
+    <property name="Column"/>
+    <property name="Name"/>
+    <property name="Abracadabra"/>
+    <set name="Rows" cascade="none">
+      <key column="Order"/>
+      <one-to-many class="OrderRow" />
+    </set>
+  </class>
+  <class name="OrderRow">
+    <id name="Id">
+      <generator class="native"/>
+    </id>
+    <property name="Name"/>
+    <many-to-one name="Order" column="Order" not-null="true"/>
+  </class>
 
-</hibernate-mapping>
+</hibernate-mapping>

src/NHibernate.Test/Tools/hbm2ddl/SchemaMetadataUpdaterTest/HeavyEntity.cs renamed to src/NHibernate.Test/Tools/hbm2ddl/SchemaMetadataUpdaterTest/Order.cs

@@ -1,3 +1,5 @@
+using System.Collections.Generic;
+
 namespace NHibernate.Test.Tools.hbm2ddl.SchemaMetadataUpdaterTest
 {
 	public class Order
@@ -8,5 +10,6 @@ public class Order
 		public string Column { get; set; }
 		public string Name { get; set; }
 		public string Abracadabra { get; set; }
+		public ISet<OrderRow> Rows { get; set; }
 	}
-}
+}

src/NHibernate.Test/Tools/hbm2ddl/SchemaMetadataUpdaterTest/OrderRow.cs

@@ -0,0 +1,9 @@
+namespace NHibernate.Test.Tools.hbm2ddl.SchemaMetadataUpdaterTest
+{
+	public class OrderRow
+	{
+		public int Id { get; set; }
+		public Order Order { get; set; }
+		public string Name { get; set; }
+	}
+}

src/NHibernate/Tool/hbm2ddl/SchemaMetadataUpdater.cs

@@ -55,6 +55,8 @@ public static void QuoteTableAndColumns(Configuration configuration)
 
 		public static void QuoteTableAndColumns(Configuration configuration, Dialect.Dialect dialect)
 		{
+			// We have to build the mappings in order to quote collection mappings that use a second pass command to be fully initialized
+			configuration.BuildMappings();
 			foreach (var cm in configuration.ClassMappings)
 			{
 				QuoteTable(cm.Table, dialect);