You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: mod.rs
    path of file: ./repos/pgrx/pgrx-sql-entity-graph/src/to_sql
    
    the code of the file until where you have to start completion: //LICENSE Portions Copyright 2019-2021 ZomboDB, LLC.
//LICENSE
//LICENSE Portions Copyright 2021-2023 Technology Concepts & Design, Inc.
//LICENSE
//LICENSE Portions Copyright 2023-2023 PgCentral Foundation, Inc. <contact@pgcentral.org>
//LICENSE
//LICENSE All rights reserved.
//LICENSE
//LICENSE Use of this source code is governed by the MIT license that can be found in the LICENSE file.
/*!

`sql = ...` fragment related macro expansion for Rust to SQL translation

> Like all of the [`sql_entity_graph`][crate] APIs, this is considered **internal**
to the `pgrx` framework and very subject to change between versions. While you may use this, please do it with caution.

*/
pub mod entity;

use std::hash::Hash;

use proc_macro2::TokenStream as TokenStream2;
use 
    
    fn to_tokens(&self, tokens: &mut TokenStream2) {
        let enabled = self.enabled;
        let callback = &self.callback;
        let content = &self.content;
        if let Some(callback_path) = callback {
            tokens.append_all(quote! {
                ::pgrx::pgrx_sql_entity_graph::ToSqlConfigEntity {
                    enabled: #enabled,
                    callback: Some(#callback_path),
                    content: None,
                }
            });
            return;
        }
        if let Some(sql) = content {
            tokens.append_all(quote! {
                ::pgrx::pgrx_sql_entity_graph::ToSqlConfigEntity {
                    enabled: #enabled,
                    callback: None,
                    content: Some(#sql),
                }
            });
            return;
        }
        tokens.append_all(quote! {
            ::pgrx::pgrx_sql_entity_graph::ToSqlConfigEntity {
                enabled: #enabled,
                callback: None,
                content: None,
            }
        });
    }