Implemented MongoDB Atlas Self-Query Retriever (#13321)

# Description 
This PR implements Self-Query Retriever for MongoDB Atlas vector store.

I've implemented the comparators and operators that are supported by
MongoDB Atlas vector store according to the section titled "Atlas Vector
Search Pre-Filter" from
https://www.mongodb.com/docs/atlas/atlas-vector-search/vector-search-stage/.

Namely:
```
allowed_comparators = [
      Comparator.EQ,
      Comparator.NE,
      Comparator.GT,
      Comparator.GTE,
      Comparator.LT,
      Comparator.LTE,
      Comparator.IN,
      Comparator.NIN,
  ]

"""Subset of allowed logical operators."""
allowed_operators = [
    Operator.AND,
    Operator.OR
]
```
Translations from comparators/operators to MongoDB Atlas filter
operators(you can find the syntax in the "Atlas Vector Search
Pre-Filter" section from the previous link) are done using the following
dictionary:
```
map_dict = {
            Operator.AND: "$and",
            Operator.OR: "$or",
            Comparator.EQ: "$eq",
            Comparator.NE: "$ne",
            Comparator.GTE: "$gte",
            Comparator.LTE: "$lte",
            Comparator.LT: "$lt",
            Comparator.GT: "$gt",
            Comparator.IN: "$in",
            Comparator.NIN: "$nin",
        }
```

In visit_structured_query() the filters are passed as "pre_filter" and
not "filter" as in the MongoDB link above since langchain's
implementation of MongoDB atlas vector
store(libs\langchain\langchain\vectorstores\mongodb_atlas.py) in
_similarity_search_with_score() sets the "filter" key to have the value
of the "pre_filter" argument.
```
params["filter"] = pre_filter
```
Test cases and documentation have also been added.

# Issue
#11616 

# Dependencies
No new dependencies have been added.

# Documentation
I have created the notebook mongodb_atlas_self_query.ipynb outlining the
steps to get the self-query mechanism working.

I worked closely with [@Farhan-Faisal](https://github.com/Farhan-Faisal)
on this PR.

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/retrievers/self_query/base.py

@@ -21,6 +21,7 @@ from langchain.retrievers.self_query.dashvector import DashvectorTranslator
 from langchain.retrievers.self_query.deeplake import DeepLakeTranslator
 from langchain.retrievers.self_query.elasticsearch import ElasticsearchTranslator
 from langchain.retrievers.self_query.milvus import MilvusTranslator
+from langchain.retrievers.self_query.mongodb_atlas import MongoDBAtlasTranslator
 from langchain.retrievers.self_query.myscale import MyScaleTranslator
 from langchain.retrievers.self_query.opensearch import OpenSearchTranslator
 from langchain.retrievers.self_query.pinecone import PineconeTranslator
@@ -36,6 +37,7 @@ from langchain.vectorstores import (
     DeepLake,
     ElasticsearchStore,
     Milvus,
+    MongoDBAtlasVectorSearch,
     MyScale,
     OpenSearchVectorSearch,
     Pinecone,
@@ -66,6 +68,7 @@ def _get_builtin_translator(vectorstore: VectorStore) -> Visitor:
         SupabaseVectorStore: SupabaseVectorTranslator,
         TimescaleVector: TimescaleVectorTranslator,
         OpenSearchVectorSearch: OpenSearchTranslator,
+        MongoDBAtlasVectorSearch: MongoDBAtlasTranslator,
     }
     if isinstance(vectorstore, Qdrant):
         return QdrantTranslator(metadata_key=vectorstore.metadata_payload_key)

libs/langchain/langchain/retrievers/self_query/mongodb_atlas.py

@@ -0,0 +1,74 @@
+"""Logic for converting internal query language to a valid MongoDB Atlas query."""
+from typing import Dict, Tuple, Union
+
+from langchain.chains.query_constructor.ir import (
+    Comparator,
+    Comparison,
+    Operation,
+    Operator,
+    StructuredQuery,
+    Visitor,
+)
+
+MULTIPLE_ARITY_COMPARATORS = [Comparator.IN, Comparator.NIN]
+
+
+class MongoDBAtlasTranslator(Visitor):
+    """Translate Mongo internal query language elements to valid filters."""
+
+    """Subset of allowed logical comparators."""
+    allowed_comparators = [
+        Comparator.EQ,
+        Comparator.NE,
+        Comparator.GT,
+        Comparator.GTE,
+        Comparator.LT,
+        Comparator.LTE,
+        Comparator.IN,
+        Comparator.NIN,
+    ]
+
+    """Subset of allowed logical operators."""
+    allowed_operators = [Operator.AND, Operator.OR]
+
+    ## Convert a operator or a comparator to Mongo Query Format
+    def _format_func(self, func: Union[Operator, Comparator]) -> str:
+        self._validate_func(func)
+        map_dict = {
+            Operator.AND: "$and",
+            Operator.OR: "$or",
+            Comparator.EQ: "$eq",
+            Comparator.NE: "$ne",
+            Comparator.GTE: "$gte",
+            Comparator.LTE: "$lte",
+            Comparator.LT: "$lt",
+            Comparator.GT: "$gt",
+            Comparator.IN: "$in",
+            Comparator.NIN: "$nin",
+        }
+        return map_dict[func]
+
+    def visit_operation(self, operation: Operation) -> Dict:
+        args = [arg.accept(self) for arg in operation.arguments]
+        return {self._format_func(operation.operator): args}
+
+    def visit_comparison(self, comparison: Comparison) -> Dict:
+        if comparison.comparator in MULTIPLE_ARITY_COMPARATORS and not isinstance(
+            comparison.value, list
+        ):
+            comparison.value = [comparison.value]
+
+        comparator = self._format_func(comparison.comparator)
+
+        attribute = comparison.attribute
+
+        return {attribute: {comparator: comparison.value}}
+
+    def visit_structured_query(
+        self, structured_query: StructuredQuery
+    ) -> Tuple[str, dict]:
+        if structured_query.filter is None:
+            kwargs = {}
+        else:
+            kwargs = {"pre_filter": structured_query.filter.accept(self)}
+        return structured_query.query, kwargs

libs/langchain/tests/unit_tests/retrievers/self_query/test_mongodb_atlas.py

@@ -0,0 +1,131 @@
+from typing import Dict, Tuple
+
+from langchain.chains.query_constructor.ir import (
+    Comparator,
+    Comparison,
+    Operation,
+    Operator,
+    StructuredQuery,
+)
+from langchain.retrievers.self_query.mongodb_atlas import MongoDBAtlasTranslator
+
+DEFAULT_TRANSLATOR = MongoDBAtlasTranslator()
+
+
+def test_visit_comparison_lt() -> None:
+    comp = Comparison(comparator=Comparator.LT, attribute="qty", value=20)
+    expected = {"qty": {"$lt": 20}}
+    actual = DEFAULT_TRANSLATOR.visit_comparison(comp)
+    assert expected == actual
+
+
+def test_visit_comparison_eq() -> None:
+    comp = Comparison(comparator=Comparator.EQ, attribute="qty", value=10)
+    expected = {"qty": {"$eq": 10}}
+    actual = DEFAULT_TRANSLATOR.visit_comparison(comp)
+    assert expected == actual
+
+
+def test_visit_comparison_ne() -> None:
+    comp = Comparison(comparator=Comparator.NE, attribute="name", value="foo")
+    expected = {"name": {"$ne": "foo"}}
+    actual = DEFAULT_TRANSLATOR.visit_comparison(comp)
+    assert expected == actual
+
+
+def test_visit_comparison_in() -> None:
+    comp = Comparison(comparator=Comparator.IN, attribute="name", value="foo")
+    expected = {"name": {"$in": ["foo"]}}
+    actual = DEFAULT_TRANSLATOR.visit_comparison(comp)
+    assert expected == actual
+
+
+def test_visit_comparison_nin() -> None:
+    comp = Comparison(comparator=Comparator.NIN, attribute="name", value="foo")
+    expected = {"name": {"$nin": ["foo"]}}
+    actual = DEFAULT_TRANSLATOR.visit_comparison(comp)
+    assert expected == actual
+
+
+def test_visit_operation() -> None:
+    op = Operation(
+        operator=Operator.AND,
+        arguments=[
+            Comparison(comparator=Comparator.GTE, attribute="qty", value=10),
+            Comparison(comparator=Comparator.LTE, attribute="qty", value=20),
+            Comparison(comparator=Comparator.EQ, attribute="name", value="foo"),
+        ],
+    )
+    expected = {
+        "$and": [
+            {"qty": {"$gte": 10}},
+            {"qty": {"$lte": 20}},
+            {"name": {"$eq": "foo"}},
+        ]
+    }
+    actual = DEFAULT_TRANSLATOR.visit_operation(op)
+    assert expected == actual
+
+
+def test_visit_structured_query_no_filter() -> None:
+    query = "What is the capital of France?"
+    structured_query = StructuredQuery(
+        query=query,
+        filter=None,
+    )
+    expected: Tuple[str, Dict] = (query, {})
+    actual = DEFAULT_TRANSLATOR.visit_structured_query(structured_query)
+    assert expected == actual
+
+
+def test_visit_structured_query_one_attr() -> None:
+    query = "What is the capital of France?"
+    comp = Comparison(comparator=Comparator.IN, attribute="qty", value=[5, 15, 20])
+    structured_query = StructuredQuery(
+        query=query,
+        filter=comp,
+    )
+    expected = (
+        query,
+        {"pre_filter": {"qty": {"$in": [5, 15, 20]}}},
+    )
+    actual = DEFAULT_TRANSLATOR.visit_structured_query(structured_query)
+    assert expected == actual
+
+
+def test_visit_structured_query_deep_nesting() -> None:
+    query = "What is the capital of France?"
+    op = Operation(
+        operator=Operator.AND,
+        arguments=[
+            Comparison(comparator=Comparator.EQ, attribute="name", value="foo"),
+            Operation(
+                operator=Operator.OR,
+                arguments=[
+                    Comparison(comparator=Comparator.GT, attribute="qty", value=6),
+                    Comparison(
+                        comparator=Comparator.NIN,
+                        attribute="tags",
+                        value=["bar", "foo"],
+                    ),
+                ],
+            ),
+        ],
+    )
+    structured_query = StructuredQuery(
+        query=query,
+        filter=op,
+    )
+    expected = (
+        query,
+        {
+            "pre_filter": {
+                "$and": [
+                    {"name": {"$eq": "foo"}},
+                    {"$or": [{"qty": {"$gt": 6}}, {"tags": {"$nin": ["bar", "foo"]}}]},
+                ]
+            }
+        },
+    )
+    actual = DEFAULT_TRANSLATOR.visit_structured_query(structured_query)
+    assert expected == actual