# `h-sql`
Let's serve a list of comments with a comment form! ```ocaml module type DB = Caqti_lwt.CONNECTION module R = Caqti_request module T = Caqti_type let list_comments = let query = R.collect T.unit T.(tup2 int string) "SELECT id, text FROM comment" in fun (module Db : DB) -> let%lwt comments_or_error = Db.collect_list query () in Caqti_lwt.or_fail comments_or_error let add_comment = let query = R.exec T.string "INSERT INTO comment (text) VALUES ($1)" in fun text (module Db : DB) -> let%lwt unit_or_error = Db.exec query text in Caqti_lwt.or_fail unit_or_error let render comments request = % comments |> List.iter (fun (_id, comment) ->

<%s comment %>

<% ); %> <%s! Dream.Tag.form ~action:"/" request %> let () = Dream.run @@ Dream.logger @@ Dream.sql_pool "sqlite3:db.sqlite" @@ Dream.sql_sessions @@ Dream.router [ Dream.get "/" (fun request -> let%lwt comments = Dream.sql list_comments request in Dream.respond (render comments request)); Dream.post "/" (fun request -> match%lwt Dream.form request with | `Ok ["text", text] -> let%lwt () = Dream.sql (add_comment text) request in let%lwt comments = Dream.sql list_comments request in Dream.respond (render comments request) | _ -> Dream.empty `Bad_Request); ] @@ Dream.not_found ```

$ dune exec --root . ./sql.exe

Try visiting [http://localhost:8080](http://localhost:8080) and leaving some comments! ![Comments](https://raw.githubusercontent.com/aantron/dream/master/docs/asset/sql.png)
Several things are going on in this example. It... - sets up the boilerplate for two SQL queries, `list_comments` and `add_comment` with [Caqti](https://paurkedal.github.io/ocaml-caqti/caqti/Caqti_connect_sig/module-type-S/module-type-CONNECTION/index.html); - defines a template, `render`, for our app's main page; - sets up a [pool of SQL connections](https://aantron.github.io/dream/#val-sql_pool) to `db.sqlite`; and - sets up two routes, one for displaying the comment list, and one for receiving new comments from our CSRF-safe form (example [**`d-form`**](../d-form#files)). We also take the opportunity to try out [`Dream.sql_sessions`](https://aantron.github.io/dream/#val-sql_sessions), which stores session data persistently in `db.sqlite`! See example [**`b-session`**](../b-session#files) for an introduction to session management.
`db.sqlite` was initialized with this schema, using the `sqlite3` command: ```sql CREATE TABLE comment ( id INTEGER PRIMARY KEY AUTOINCREMENT, text NOT NULL); CREATE TABLE dream_session ( key TEXT NOT NULL PRIMARY KEY, id TEXT NOT NULL, expires_at REAL NOT NULL, payload TEXT NOT NULL ); ``` We also had to make an addition to our Dune file:

(executable
 (name sql)
 (libraries caqti-driver-sqlite3 dream)
 (preprocess (pps lwt_ppx)))

(rule
 (targets sql.ml)
 (deps sql.eml.ml)
 (action (run dream_eml %{deps} --workspace %{workspace_root})))

SQLite is good for small-to-medium sites and examples. For a larger site, microservices, or other needs, you can switch, for example, to PostgreSQL by... - running PostgreSQL, typically in a [Docker container](https://hub.docker.com/_/postgres/); - changing the connection URI to `postgres://user:password@host:port`; - using `caqti-driver-postgres`; - replacing `INTEGER PRIMARY KEY AUTOINCREMENT` by `SERIAL PRIMARY KEY`. A good program for examining the database locally is [Beekeeper Studio](https://www.beekeeperstudio.io/). Dream might also integrate an optional hosted database UI in the future, and you could choose to serve it at some route.
See - [`Caqti_connect_sig.S.CONNECTION`](https://paurkedal.github.io/ocaml-caqti/caqti/Caqti_connect_sig/module-type-S/module-type-CONNECTION/index.html) for Caqti's statement runners. These are the fields of the module `Db` in the example. - [`Caqti_request`](https://paurkedal.github.io/ocaml-caqti/caqti/Caqti_request/) sets up prepared statements. - [`Caqti_type`](https://paurkedal.github.io/ocaml-caqti/caqti/Caqti_type/) is used to specify the types of statement arguments and results.

**Next steps:** - [**`i-graphql`**](../i-graphql#files) handles GraphQL queries and serves GraphiQL. - [**`j-stream`**](../j-stream#files) streams response bodies to clients.
[Up to the tutorial index](../#readme)