feat: add support for DBML Checks block syntax

[frenzzy]

Fixes #49

Summary

Add parser support for the DBML Checks block syntax, which was introduced in the official DBML specification (v5.0.0). This allows parsing DBML files generated by @dbml/cli sql2dbml that include CHECK constraint definitions.

Problem

When using sql2dbml to convert PostgreSQL schemas that contain CHECK constraints, the generated DBML includes Checks blocks:

Table "user_role" {
  id uuid [pk]
  role_id uuid
  platform_role_id uuid

  Checks {
    `(num_nonnulls(role_id, platform_role_id) = 1)` [name: 'user_role_ck']
  }
}

This causes dbml-renderer to fail with:

Could not parse input at line X. Expected "'", "'''", "\"", or whitespace but "`" found.

Solution

Added parser support following the existing Indices block pattern:

Grammar (src/dbml.pegjs)

TableItem =
  Column
  / Indices
  / Checks  // Added
  / option:Option { return { type: "option", option }; }

Checks = "Checks"i __ "{" __ checks:ChecksList __ "}" { return { type: "checks", checks }; }
ChecksList = (head:CheckItem tail:(EOL __ check:CheckItem { return check; })* { return [head, ...tail]; })?
CheckItem = expression:Function _ settings:Settings? { return { expression, settings } }

Types (src/types.ts)

export const TableChecks = z.object({
  type: z.literal("checks"),
  checks: z.array(
    z.object({
      expression: z.string(),
      settings: Settings.nullable().transform((v) => v || {}),
    }),
  ),
});

Checker (src/checker.ts)

• Added checks property to NormalizedTable
• Added checks extraction using existing extract() helper

Implementation Notes

• Checks are parsed and stored in the AST but not rendered visually (same as index details), since they are constraint metadata
• The implementation exactly mirrors the existing Indices pattern for consistency
• Tested against real-world PostgreSQL schema with multiple Checks blocks

Test Coverage

Added examples/checks.dbml test file covering:

• Simple check constraints (price > 0)
• Named checks with settings ([name: 'constraint_name'])
• Complex expressions with AND/OR logic
• Multiple checks per table
• Checks combined with indexes and references

Test count: 100 → 104 (4 new tests added)

References

• DBML Check Syntax: https://dbml.dbdiagram.io/docs/#check-definition
• @dbml/core Check Support: Add support for custom constraints  holistics/dbml#484 (implemented in PR #765, released in v5.0.0)

Test Plan

• All 104 tests pass
• npm run format:check passes
• npm run build passes
• Successfully parses DBML files with Checks blocks
• Generates valid SVG/DOT output from schemas containing Checks
• No regressions in existing functionality

[frenzzy]

While waiting for this PR to be merged, I've published a temporary fork to npm:

Package: @frenzzy/dbml-renderer
Version: 1.0.31-checks.0
Tag: checks

Usage

# Using npx
npx @frenzzy/dbml-renderer@checks -i schema.dbml -o schema.svg

# Or install as dependency
npm install @frenzzy/dbml-renderer@checks

This is a drop-in replacement until the official package is updated.