feat(preconditions): add support for isEventQLTrue preconditions

README.md

@@ -114,6 +114,23 @@ match result {
 
 *Note that according to the CloudEvents standard, event IDs must be of type string.*
 
+#### Using the `IsEventQLTrue` precondition
+
+If you want to write events depending on an EventQL query, use the `IsEventQLTrue` precondition to create a precondition and pass it in a vector as the second argument:
+
+```rust
+let result = client.write_events(
+  vec![event.clone()],
+  vec![Precondition::IsEventQLTrue {
+    query: "FROM e IN events WHERE e.type == 'io.eventsourcingdb.library.book-borrowed' PROJECT INTO COUNT() < 10".to_string(),
+  }],
+).await;
+match result {
+  Ok(written_events) => // ...
+  Err(err) => // ...
+}
+```
+
 ### Reading Events
 
 To read all events of a subject, call the `read_events` function with the subject and an options object. Set the `recursive` option to `false`. This ensures that only events of the given subject are returned, not events of nested subjects.

src/client/precondition.rs

@@ -19,4 +19,10 @@ pub enum Precondition {
         #[serde(rename = "eventId")]
         event_id: String,
     },
+    /// Check if an EventQL query returns true
+    #[serde(rename = "isEventQlTrue")]
+    IsEventQLTrue {
+        /// The EventQL query to check
+        query: String,
+    },
 }

tests/write_events.rs

@@ -212,6 +212,26 @@ async fn write_events_with_is_subject_on_event_id_condition_on_empty_subject() {
     assert!(result.is_err(), "Expected an error, but got: {:?}", result);
 }
 
+#[tokio::test]
+async fn write_events_with_is_eventql_true_condition() {
+    let container = Container::start_default().await.unwrap();
+    let client = container.get_client().await.unwrap();
+
+    let event_candidates = vec![
+        create_test_eventcandidate("/test/42", json!({"value": 1})),
+        create_test_eventcandidate("/test/42", json!({"value": 1})),
+    ];
+    let result = client
+        .write_events(
+            event_candidates.clone(),
+            vec![Precondition::IsEventQLTrue {
+                query: "FROM e IN events PROJECT INTO COUNT() == 0".to_string(),
+            }],
+        )
+        .await;
+    assert!(result.is_ok(), "Expected an ok, but got: {:?}", result);
+}
+
 #[tokio::test]
 async fn write_events_with_is_subject_on_event_id_condition_on_non_empty_subject_correct_id() {
     let container = Container::start_default().await.unwrap();