QueryTrait::maybe: a general purpose query customizer (#1415)

* Added `QueryTrait::maybe`

* Make the API better

* Docs

* Update src/query/traits.rs

Co-authored-by: Chris Tsang <chris.2y3@outlook.com>

src/query/traits.rs

@@ -23,4 +23,35 @@ pub trait QueryTrait {
             self.as_query().build_any(query_builder.as_ref()),
         )
     }
+
+    /// Apply an operation on the [QueryTrait::QueryStatement] if the given `Option<T>` is `Some(_)`
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// use sea_orm::{entity::*, query::*, tests_cfg::cake, DbBackend};
+    ///
+    /// assert_eq!(
+    ///     cake::Entity::find()
+    ///         .apply_if(Some(3), |mut query, v| {
+    ///             query.filter(cake::Column::Id.eq(v))
+    ///         })
+    ///         .apply_if(Some(100), QuerySelect::limit)
+    ///         .apply_if(None, QuerySelect::offset) // no-op
+    ///         .build(DbBackend::Postgres)
+    ///         .to_string(),
+    ///     r#"SELECT "cake"."id", "cake"."name" FROM "cake" WHERE "cake"."id" = 3 LIMIT 100"#
+    /// );
+    /// ```
+    fn apply_if<T, F>(self, val: Option<T>, if_some: F) -> Self
+    where
+        Self: Sized,
+        F: FnOnce(Self, T) -> Self,
+    {
+        if let Some(val) = val {
+            if_some(self, val)
+        } else {
+            self
+        }
+    }
 }