server_action_add_field.qmd

---
title: "`add_field` Action"
description: "Implement the add_field DoAction to support adding fields to STRUCT columns in your Arrow Flight server tables."
---

The `add_field` action adds a new field to a nested `STRUCT` column within a table. This action is invoked when executing an `ALTER TABLE ... ADD COLUMN` SQL statement that targets a field within a struct.

## SQL Example

```sql
-- Add a field to a nested struct column
ALTER TABLE example.main.employees ADD COLUMN address.city VARCHAR;
ALTER TABLE example.main.employees ADD COLUMN IF NOT EXISTS metadata.created_by VARCHAR;
```

## Input Parameters

The action receives a single `msgpack`-serialized parameter:

```c++
// The base class for all alter parameters.
struct AirportAlterBase
{
  //! Catalog name to alter
  std::string catalog;
  //! Schema name to alter
  std::string schema;
  //! Entry name to alter
  std::string name;
  bool ignore_not_found;
};

struct AirportAlterTableAddFieldParameters : AirportAlterBase
{
  // Contains the path to the field and the Arrow type for the new field.
  std::string column_schema;
  bool if_field_not_exists;

  MSGPACK_DEFINE_MAP(catalog, schema, name, ignore_not_found, if_field_not_exists, column_schema);
};

```

## Return Value

The action must return a single [`FlightInfo`](https://github.com/apache/arrow/blob/ac1f05f28e18e85ee55cf7aaf3c8ae1ffe0e92d7/format/Flight.proto#L275) structure representing the modified table with the new field added to the struct column. The `app_metadata` field must be populated appropriately to identify the flight as a table.