db.rs
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	//! # Utilities for database.
	//!
	//! This module provides helpers to interact with a PostgreSQL database.
	//! Functions expect to work with an executor from sqlx crate.

	use std::future::Future;

	use async_scoped::TokioScope;
	use sqlx::PgPool;
	use sqlx::postgres::PgPoolOptions;

	static QUERIES_SEPARATOR: &str = "\n\n\n";

	pub struct TableInitializationArgs {
	pub table_name: String,
	pub create_table: bool,
	pub overwrite_table: bool,
	}

	pub trait ToTableInitializationArgs {
	fn to_table_initialization_args(&self) -> TableInitializationArgs;
	}

	pub async fn connect_to_db (database_url: &str) -> PgPool {
	PgPoolOptions::new()
	.max_connections(3)
	.connect(database_url)
	.await
	.expect("Can't connect to database.")
	}

	pub async fn is_table_exists (pool: &PgPool, table: &str) -> bool {
	let query = r#"
	SELECT EXISTS (
	SELECT FROM
	pg_tables
	WHERE
	schemaname = 'public' AND
	tablename = $1
	);
	"#;

	let result: (bool,) = sqlx::query_as(query)
	.bind(table)
	.fetch_one(pool)
	.await
	.expect("Can't check if table exists.");

	result.0
	}

	pub async fn is_table_empty (pool: &PgPool, table: &str) -> bool {
	let query = r#"
	SELECT EXISTS (
	SELECT 1 FROM %%table%%
	);
	"#.replace("%%table%%", table);

	let result: (bool,) = sqlx::query_as(&query)
	.fetch_one(pool)
	.await
	.expect("Can't check if table is empty.");

	!result.0
	}

	pub async fn truncate_table (pool: &PgPool, table: &str) {
	let query = format!("TRUNCATE TABLE {} RESTART IDENTITY;", table);

	sqlx::query(&query)
	.bind(table)
	.execute(pool)
	.await
	.expect("Can't truncate table.");
	}

	pub async fn initialize_table<F, T> (
	pool: &PgPool,
	callback: F,
	args: T
	) -> Result<(), String>
	where F: Future, T: ToTableInitializationArgs
	{
	let args = args.to_table_initialization_args();
	if is_table_exists(pool, &args.table_name).await {
	if is_table_empty(&pool, &args.table_name).await {
	return Ok(());
	}

	if args.overwrite_table {
	truncate_table(&pool, &args.table_name).await;
	return Ok(());
	}

	return Err(format!(
	"Table {} already exists and contains rows. To overwrite it, run the import tool with -t option.",
	&args.table_name
	));
	}

	if args.create_table {
	callback.await;
	return Ok(());
	}

	Err(format!(
	"Table {} doesn't exist. To create it, run the import tool with -c option.",
	&args.table_name
	))
	}

	pub async fn run_multiple_queries(pool: &PgPool, queries: &str) {
	for query in queries.split(QUERIES_SEPARATOR) {
	sqlx::query(&query)
	.execute(pool)
	.await
	.expect("Can't run SQL query.");
	}
	}

	pub fn run_multiple_queries_groups (pool: &PgPool, queries_groups: &Vec<String>) {
	let n = queries_groups.len();
	TokioScope::scope_and_block(\|scope\| {
	for i in 0..n {
	scope.spawn(
	run_multiple_queries(pool, &queries_groups[i])
	)
	}
	});
	}

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