Introduction

What is MetaDbDiff?

MetaDbDiff is a database metadata comparison and DDL migration script generation engine for Delphi and Lazarus. It is entirely decoupled from any transactional ORM layer: it has one job — compare two schema descriptions and emit the SQL ALTER/CREATE/DROP statements required to reconcile them.

It serves two use-cases:

1. Model-to-Database sync — You decorate Delphi Pascal entity classes with attributes ([Table], [Column], [PrimaryKey], [ForeignKey], [Indexe], etc.) and MetaDbDiff reads them via RTTI, extracts the live schema from the database through a DataEngine connection, then computes the diff.

2. Database-to-Database sync — You provide two IDBConnection handles (source/master and target). MetaDbDiff extracts both schemas and generates the DDL to evolve the target to match the source.

Design goals

• Surgical DDL: no full-schema drops. Each command is as narrow as possible — ALTER COLUMN when only a type changes, DROP INDEX + CREATE INDEX when uniqueness changes, etc.
• Driver isolation: every database engine (Firebird, PostgreSQL, MySQL, MSSQL, Oracle, SQLite, Interbase, AbsoluteDB) implements TDDLSQLGenerator and registers itself in TDDLRegister. The core never references a dialect directly.
• Feature flags: dialects declare their capabilities via TSupportedFeatures (Sequences, ForeignKeys, Checks, Views, Triggers). The comparison engine guards every optional block with a feature check.
• Transaction safety: DDL commands are batched through IDBConnection.AddScript / ExecuteScripts / Commit; if any step raises the connection is rolled back.

Core internal model (MIK layer)

All schema metadata — whether extracted from a live database or derived from RTTI entity classes — is translated into the MIK (MetaInfo Kind) object model before comparison:

Class	Represents
TCatalogMetadataMIK	A full database catalog (tables, sequences, views)
TTableMIK	A single table with its columns, PK, indexes, FKs, checks, triggers
TColumnMIK	A column with name, type, size, precision, nullability, charset, position, default value
TPrimaryKeyMIK	A primary key with its constituent column set
TForeignKeyMIK	A foreign key with source/target column maps and referential actions
TIndexeKeyMIK	An index with its column set and uniqueness flag
TCheckMIK	A CHECK constraint
TTriggerMIK	A trigger (script)
TViewMIK	A database view
TSequenceMIK	A sequence / generator

Relation to Janus ORM

MetaDbDiff is the structural foundation used by the Janus ORM for its BuildDatabase / auto-migration feature. Janus registers its entity classes in the MetaDbDiff mapping registry; MetaDbDiff then computes and optionally executes the required DDL at startup. You can also use MetaDbDiff standalone — no Janus dependency is required.

Dependency

MetaDbDiff requires DataEngine (github.com/ModernDelphiWorks/DataEngine) for the IDBConnection abstraction layer. DataEngine provides driver wrappers for FireDAC, DBX, Zeos, and others.