Best practices for specifying the database connection in SQL exercises

[gadenbuie]

We should think through the best approach for using SQL exercises in learnr documents. Currently, the following small tutorial won't work

---
title: "SQL"
output: learnr::tutorial
runtime: shiny_prerendered
---

## Using SQL

```{r setup}
library(learnr)
```

```{sql sql-ex, connection = starwarsdb::starwars_connect(), exercise = TRUE}
select * from people;
```

because the connection chunk option is evaluated and serialized into

connection = new("duckdb_connection", dbdir = character(0), 
  conn_ref = <pointer: 0x7fe936fc8080>, driver = new("duckdb_driver", 
    database_ref = <pointer: 0x7fe936f7a9f0>, dbdir = ":memory:", 
    read_only = FALSE), debug = FALSE)

which is broken by the <pointer: 0x7fe936fc8080>.

When this happens, the only way is to create the connection in the global setup chunk and reference its object name as a character string. Even using the connection object name unquoted will cause it to be serialized.

---
title: "SQL"
output: learnr::tutorial
runtime: shiny_prerendered
---

## Using SQL

```{r setup}
library(learnr)
con <- starwarsdb::starwars_connect()
```

```{sql sql-ex, connection = "con", exercise = TRUE}
select * from people;
```

We might want to protect this option from serialization. I also wonder what would be the best approach for handling connection pooling or serving tutorials for many users at once from the same host.