diff --git a/lib/ddl.js b/lib/ddl.js
index 2ae688a..ae4874f 100644
--- a/lib/ddl.js
+++ b/lib/ddl.js
@@ -98,15 +98,15 @@ class PgSchema extends DatabaseSchema {
   }
 }
 
+DatabaseSchema.implementations.pg = PgSchema;
+
 const generate = async (schemaPath, outputPath) => {
   const schema = await new PgSchema(schemaPath);
-  await schema.preprocess();
   await schema.generate(outputPath);
 };
 
 const migrate = async schemaPath => {
   const schema = await new PgSchema(schemaPath);
-  await schema.preprocess();
   await schema.migrate();
 };
 
diff --git a/lib/schema-db.js b/lib/schema-db.js
index 6c43b1f..ca51578 100644
--- a/lib/schema-db.js
+++ b/lib/schema-db.js
@@ -29,6 +29,7 @@ class DatabaseSchema {
       const schemaName = path.basename(file.name, '.js');
       this.entities.set(schemaName, schema);
     }
+    await this.preprocess();
     return this;
   }
 
@@ -102,7 +103,9 @@ class DatabaseSchema {
     }
     if (version === 0) return null;
     const previousPath = path.join(historyPath, previousName);
-    return new DatabaseSchema(previousPath);
+    const { driver } = this.database;
+    const SchemaClass = DatabaseSchema.implementations[driver];
+    return new SchemaClass(previousPath);
   }
 
   async migrate() {
@@ -127,4 +130,6 @@ class DatabaseSchema {
   }
 }
 
+DatabaseSchema.implementations = {};
+
 module.exports = { DatabaseSchema };