import { Meta } from "@storybook/addon-docs";
import { CodePreview } from "./CodePreview";

<Meta title="6. Transformations" />

# 6. Transformations

Transformations let you intercept and modify query structures at SQL generation time. This is useful for enforcing ACL rules, dynamic table routing, default filters, or modifying mutations without altering each query manually.

## Usage

When calling `toSQL`, provide transformation functions:
- `transformTable`: Alters table references.
- `transformSelectQuery`: Modifies `SELECT` queries.
- `transformInsertMutation`: Adjusts `INSERT` statements.
- `transformUpdateMutation`: Modifies `UPDATE` operations.
- `transformDeleteMutation`: Restricts `DELETE` queries.

## Transforming Table References

This example replaces a table reference with a subquery:

<CodePreview
  code={`const baseQuery = Q.select().from("foo", "f1");
const joinQuery = baseQuery.leftJoin(Q.table("foo2", "f2"), Cond.columnEqual("f1.id", "f2.id"));

return joinQuery.toSQL(Q.flavors.mysql, {
  transformTable: (table) => {
    if (table === "foo") {
      return Q.select()
        .from(table, "f")
        .join(Q.table("bar", "b"), Cond.columnEqual("f.id", "b.id"));
    }
    return table;
  },
});
`}/>

## Modifying `SELECT` Queries

You can modify fields in `SELECT` queries dynamically:

<CodePreview
  code={`const baseQuery = Q.select().from("foo");
const unionQuery = baseQuery.union(Q.select().from("blah"));

return unionQuery.toSQL(Q.flavors.mysql, {
  transformSelectQuery: (select) => {
    return select.addField("f1.name", "f1name");
  },
});
`}/>

## Altering `INSERT` Statements

Automatically add fields like `createdAt`:

<CodePreview
  code={`const insertQuery = Q.insert("foo").values([{ id: 1, name: "foo" }]);

return insertQuery.toSQL(Q.flavors.mysql, {
  transformInsertMutation: (q) => {
    const values = q.allValues().map((v) => ({ ...v, createdAt: "YYYY-MM-DD" }));
    return q.removeAllValues().values(values);
  },
});
`}/>

## Modifying `UPDATE` Operations

Ensure updates meet specific conditions:

<CodePreview
  code={`const updateQuery = Q.update("foo").set({ firstName: "john" });

return updateQuery.toSQL(Q.flavors.mysql, {
  transformUpdateMutation: (q) => {
    return q.set({ firstName: "jane" }).where(Cond.equal("lastName", "doe"));
  },
});
`}/>

## Restricting `DELETE` Queries

Add automatic conditions to `DELETE` queries:

<CodePreview
  code={`const deleteQuery = Q.delete("foo");

return deleteQuery.toSQL(Q.flavors.mysql, {
  transformDeleteMutation: (q) => {
    return q.where(Cond.equal("owner", "12345"));
  },
});
`}/>

## Common Use Cases

1. **ACL Enforcement**  
   Automatically append conditions for access control.

2. **Sharding**  
   Dynamically redirect queries to the appropriate database partition.

3. **Data Auditing**  
   Add fields like `createdAt` to `INSERT` statements automatically.

4. **Soft Deletes**  
   Transform `DELETE` into an `UPDATE` setting a `deletedAt` timestamp.

By applying these transformations, query logic remains centralized and adaptable without needing individual query modifications.