Add Equals operator.

Author: WaVEV

django_mongodb_backend/expressions/search.py

@@ -156,3 +156,44 @@ def search_operator(self, compiler, connection):
         if self.score is not None:
             params["score"] = self.score.as_mql(compiler, connection)
         return {"equals": params}
+
+
+class SearchExists(SearchExpression):
+    """
+    Atlas Search expression that matches documents where a field exists.
+
+    This expression uses the `exists` operator to check whether a given
+    path is present in the document. Useful for filtering documents that
+    include (or exclude) optional fields.
+
+    Example:
+        SearchExists("metadata__author")
+
+    Args:
+        path: The document path to check (as string or expression).
+        score: Optional expression to modify the relevance score.
+
+    Reference: https://www.mongodb.com/docs/atlas/atlas-search/exists/
+    """
+
+    def __init__(self, path, score=None):
+        self.path = cast_as_field(path)
+        self.score = score
+        super().__init__()
+
+    def get_search_fields(self, compiler, connection):
+        return {self.path.as_mql(compiler, connection, as_path=True)}
+
+    def get_source_expressions(self):
+        return [self.path]
+
+    def set_source_expressions(self, exprs):
+        (self.path,) = exprs
+
+    def search_operator(self, compiler, connection):
+        params = {
+            "path": self.path.as_mql(compiler, connection, as_path=True),
+        }
+        if self.score is not None:
+            params["score"] = self.score.as_mql(compiler, connection)
+        return {"exists": params}