If it's a has many relation, you current can't filter by related entity's column.
it's possible to expose this via an API, for now you can do the following:

let users = user::Entity::find().all(db).await?;
let activities = users
    .load_many(
        user_activities::Column::SomeColumn.eq(some_value)),
        db
    )
    .await?;

users::Entity::load().with(user_activities::Entity::find().filter(Column::SomeColumn.eq(some_value))))

the trait bound ... expected Entity, found sea_orm::Select<Entity> (rustc E0277)

Or another way is impl

pub fn has_related<R, C>(mut self, _: R, condition: C) -> Self
    where
        R: EntityTrait,
        E: Related<R>,
        C: IntoCondition,

for EntityLoader