Nix auto-column dropping, add simple concrete migrations that can be used to

easily register a column drop or a column rename should the need arise.

src/java/com/samskivert/jdbc/depot/DepotMarshaller.java

@@ -714,12 +714,8 @@ public class DepotMarshaller<T>
             ctx.invoke(new Modifier.Simple(indexdef));
         }
 
-        // remove any extra columns
-        for (String column : columns) {
-            log.info("Dropping old column '" + column + "' from " + getTableName());
-            String query = "alter table " + getTableName() + " drop index " + column;
-            ctx.invoke(new Modifier.Simple(query));
-        }
+        // we do not hand remove columns but rather require that EntityMigration.Drop records be
+        // registered by hand to avoid accidentally causin the loss of data
 
         // remove any extra indices
         for (String index : indices) {

src/java/com/samskivert/jdbc/depot/EntityMigration.java

@@ -20,6 +20,13 @@
 
 package com.samskivert.jdbc.depot;
 
+import java.sql.Connection;
+import java.sql.SQLException;
+import java.sql.Statement;
+import java.util.HashMap;
+
+import static com.samskivert.jdbc.depot.Log.log;
+
 /**
  * These can be registered with the {@link PersistenceContext} to effect hand-coded migrations
  * between entity versions. The modifier should override {@link #invoke} to perform its
@@ -28,6 +35,63 @@ package com.samskivert.jdbc.depot;
  */
 public abstract class EntityMigration extends Modifier
 {
+    /**
+     * A convenient migration for dropping a column from an entity.
+     */
+    public static class Drop extends EntityMigration
+    {
+        public Drop (int targetVersion, String columnName) {
+            super(targetVersion);
+            _columnName = columnName;
+        }
+
+        public int invoke (Connection conn) throws SQLException {
+            Statement stmt = conn.createStatement();
+            try {
+                log.info("Dropping '" + _columnName + "' from " + _tableName);
+                return stmt.executeUpdate(
+                    "alter table " + _tableName + " drop column " + _columnName);
+            } finally {
+                stmt.close();
+            }
+        }
+
+        protected String _columnName;
+    }
+
+    /**
+     * A convenient migration for renaming a column in an entity.
+     */
+    public static class Rename extends EntityMigration
+    {
+        public Rename (int targetVersion, String oldColumnName, String newColumnName) {
+            super(targetVersion);
+            _oldColumnName = oldColumnName;
+            _newColumnName = newColumnName;
+        }
+
+        public int invoke (Connection conn) throws SQLException {
+            Statement stmt = conn.createStatement();
+            try {
+                log.info("Changing '" + _oldColumnName + "' to '" + _newColumnDef + "' in " +
+                         _tableName);
+                return stmt.executeUpdate(
+                    "alter table " + _tableName + " change column " + _oldColumnName + " " +
+                    _newColumnDef);
+            } finally {
+                stmt.close();
+            }
+        }
+
+        protected void init (String tableName, HashMap<String,FieldMarshaller> marshallers)
+        {
+            super.init(tableName, marshallers);
+            _newColumnDef = marshallers.get(_newColumnName).getColumnDefinition();
+        }
+
+        protected String _oldColumnName,  _newColumnName, _newColumnDef;
+    }
+
     /**
      * If this method returns true, this migration will be run <b>before</b> the default
      * migrations, if false it will be run after.
@@ -39,12 +103,29 @@ public abstract class EntityMigration extends Modifier
 
     /**
      * When an Entity is being migrated, this method will be called to check whether this migration
-     * should be run.
+     * should be run. The default implementation runs as long as the currentVersion is less than
+     * the target version supplied to the migration at construct time.
      */
-    public abstract boolean shouldRunMigration (int currentVersion, int targetVersion);
+    public boolean shouldRunMigration (int currentVersion, int targetVersion)
+    {
+        return (currentVersion < _targetVersion);
+    }
 
-    protected EntityMigration ()
+    protected EntityMigration (int targetVersion)
     {
         super(null);
+        _targetVersion = targetVersion;
     }
+
+    /**
+     * This is called to provide the migration with the name of the entity table and access to its
+     * field marshallers prior to being invoked.
+     */
+    protected void init (String tableName, HashMap<String,FieldMarshaller> marshallers)
+    {
+        _tableName = tableName;
+    }
+
+    protected int _targetVersion;
+    protected String _tableName;
 }