Today I Learned

It was actually last night, but close enough

_posts/2020-09-15-surprise-nullable-types-diesel.md

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+---
+layout: post
+title: "Trait not bound errors with Diesel"
+tags:
+- rust
+- diesel
+- postgresql
+---
+
+Recently I have been exploring using [Diesel](https://diesel.rs) for a
+simple Rust web application. I quickly ran into a very confusing `trait
+bound` error, listed below when integrating with `chrono`. It took me a
+while to understand and fix the error, which I thought I should write down
+for later!
+
+
+The error only started showing up when I queried a specific model:
+```
+error[E0277]: the trait bound `DateTime<Utc>: FromSql<diesel::sql_types::Nullable<diesel::sql_types::Timestamptz>, Pg>` is not satisfied
+  --> src/main.rs:85:64
+   |
+85 |         let choices: Vec<Choice> = Choice::belonging_to(&poll).get_results(&pgconn).expect("Failed to get relations");
+   |                                                                ^^^^^^^^^^^ the trait `FromSql<diesel::sql_types::Nullable<diesel::sql_types::Timestamptz>, Pg>` is not implemented for `DateTime<Utc>`
+   |
+   = help: the following implementations were found:
+             <DateTime<Utc> as FromSql<diesel::sql_types::Timestamptz, Pg>>
+   = note: required because of the requirements on the impl of `Queryable<diesel::sql_types::Nullable<diesel::sql_types::Timestamptz>, Pg>` for `DateTime<Utc>`
+   = note: required because of the requirements on the impl of `Queryable<(diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Integer, diesel::sql_types::Nullable<diesel::sql_types::Timestamptz>), Pg>` for `(i32, std::string::String, i32, DateTime<Utc>)`
+   = note: required because of the requirements on the impl of `Queryable<(diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Integer, diesel::sql_types::Nullable<diesel::sql_types::Timestamptz>), Pg>` for `models::Choice`
+   = note: required because of the requirements on the impl of `LoadQuery<PooledConnection<ConnectionManager<PgConnection>>, models::Choice>` for `diesel::query_builder::SelectStatement<schema::choices::table, query_builder::select_clause::DefaultSelectClause, query_builder::distinct_clause::NoDistinctClause, query_builder::where_clause::WhereClause<diesel::expression::operators::Eq<schema::choices::columns::poll_id, diesel::expression::bound::Bound<diesel::sql_types::Integer, &i32>>>>`
+
+error: aborting due to previous error; 1 warning emitted
+```
+
+
+I stumbled into [this closed issue](https://github.com/diesel-rs/diesel/issues/2445), which didn't really help me, but it did inspire me to review the `src/schema.rs`:
+
+```rust
+table! {
+    votes (id) {
+        id -> Int4,
+        voter -> Text,
+        choice_id -> Int4,
+        poll_id -> Int4,
+        created_at -> Nullable<Timestamptz>,
+    }
+}
+```
+
+The `created_at` column is what is causing issues for me, and I noticed that it's wrapped in a `Nullable`. Meanwhile, my struct looks like:
+
+```rust
+#[derive(Associations, Debug, Identifiable, Queryable, Serialize)]
+#[belongs_to(Poll)]
+pub struct Choice {
+    id: i32,
+    details: String,
+    poll_id: i32,
+    created_at: DateTime<Utc>,
+}
+```
+
+
+Turns out the `Nullable` wrapper around the `Timestamptz` SQL type was causing behavior that I didn't want. The underlying problem ended up being that my table definition didn't include a `NOT NULL` despite including a `DEFAULT NOW()`.
+
+Naturally, altering the statement to be `created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()` caused my compilation error to disappear, fun!
+