No duplicate tables.

src/lib.rs

@@ -1,4 +1,4 @@
-use async_graphql::{Context, EmptySubscription, Object, Result, Schema};
+use async_graphql::{Context, EmptySubscription, Error, Object, Result, Schema};
 use async_std::sync::RwLock;
 use serde_json;
 
@@ -52,12 +52,17 @@ struct Mutation;
 impl Mutation {
     async fn create_table(&self, ctx: &Context<'_>, name: String) -> Result<Option<Table>> {
         let mut tables = ctx.data::<RwLock<Vec<Table>>>().unwrap().write().await;
+        match tables.binary_search_by(|t| t.name.cmp(&name)) {
+            Ok(_) => Err(Error::new(format!("Table {} already exists.", &name))),
+            Err(_) => {
                 let output = Table::new(name).await;
                 tables.push(output.clone());
                 tables.sort_by_key(|k| k.name.clone());
                 Ok(Some(output))
             }
         }
+    }
+}
 
 #[derive(Clone)]
 pub struct Database {
@@ -143,7 +148,16 @@ mod mutations {
             .execute(r#"mutation {createTable(name: "william"){name}}"#)
             .await;
         let expected = r#"{"data":{"createTable":{"name":"william"}}}"#;
-        println!("{}", &db.schema.sdl());
+        support::compare(&db, &output, &expected);
+    }
+
+    #[async_std::test]
+    async fn cannot_add_duplicate_table() {
+        let db = Database::new();
+        let qry = r#"mutation {createTable(name: "gadzoo"){name}}"#;
+        db.execute(&qry).await;
+        let output = db.execute(qry).await;
+        let expected = r#"{"data":null,"errors":[{"message":"Table gadzoo already exists.","locations":[{"line":1,"column":11}],"path":["createTable"]}]}"#;
         support::compare(&db, &output, &expected);
     }
 }