milvus: fixed bug when using partition key and dynamic fields together (#25028)

**Description:**

This PR fixes a bug where if `enable_dynamic_field` and
`partition_key_field` are enabled at the same time, a pymilvus error
occurs.

Milvus requires the partition key field to be a full schema defined
field, and not a dynamic one, so it will throw the error "the specified
partition key field {field} not exist" when creating the collection.

When `enabled_dynamic_field` is set to `True`, all schema field creation
based on `metadatas` is skipped. This code now checks if
`partition_key_field` is set, and creates the field.

Integration test added.

**Twitter handle:** StuartMarshUK

---------

Co-authored-by: Stuart Marsh <stuart.marsh@qumata.com>
Co-authored-by: Erick Friis <erick@langchain.dev>

libs/partners/milvus/langchain_milvus/vectorstores/milvus.py

@@ -433,7 +433,14 @@ class Milvus(VectorStore):
         #         ...
         # ```
         if self.enable_dynamic_field:
-            pass
+            # If both dynamic fields and partition key field are enabled
+            if self._partition_key_field is not None:
+                # create the partition field
+                fields.append(
+                    FieldSchema(
+                        self._partition_key_field, DataType.VARCHAR, max_length=65_535
+                    )
+                )
         elif self._metadata_field is not None:
             fields.append(FieldSchema(self._metadata_field, DataType.JSON))
         else:

libs/partners/milvus/tests/integration_tests/vectorstores/test_milvus.py

@@ -1,4 +1,5 @@
 """Test Milvus functionality."""
+
 from typing import Any, List, Optional
 
 import pytest
@@ -274,6 +275,34 @@ def test_milvus_metadata_field() -> None:
     }
 
 
+def test_milvus_enable_dynamic_field_with_partition_key() -> None:
+    """
+    Test end to end construction and enable dynamic field
+    with partition_key_field
+    """
+    texts = ["foo", "bar", "baz"]
+    metadatas = [{"id": i, "namespace": f"name_{i}"} for i in range(len(texts))]
+
+    docsearch = _milvus_from_texts(
+        metadatas=metadatas, enable_dynamic_field=True, partition_key_field="namespace"
+    )
+
+    # filter on a single namespace
+    output = docsearch.similarity_search("foo", k=10, expr="namespace == 'name_2'")
+    assert len(output) == 1
+
+    # without namespace filter
+    output = docsearch.similarity_search("foo", k=10)
+    assert len(output) == 3
+
+    assert set(docsearch.fields) == {
+        docsearch._primary_field,
+        docsearch._text_field,
+        docsearch._vector_field,
+        docsearch._partition_key_field,
+    }
+
+
 # if __name__ == "__main__":
 #     test_milvus()
 #     test_milvus_vector_search()