diff --git a/GUIDE.md b/GUIDE.md
index 7849585..b0aaab1 100644
--- a/GUIDE.md
+++ b/GUIDE.md
@@ -572,6 +572,71 @@ If `proxy_headers` is a callable, it will be called with single argument (`conte
 If `proxy_headers` is `None` or `False`, no headers are proxied to the other service.
 
 
+## Fields dependencies
+
+In situations where field depends on data from sibling fields in order to be resolved, `ProxySchema` can be configured to include those additional fields in root value query sent to remote schema.
+
+Below example pulls a remote schema that defines `Product` type, extends this type with `image: String` field, and then uses `ProxySchema.add_field_dependencies` to configure `{ metadata { thumb} }` as additional fields to retrieve when `image` field is queried. It also includes custom resolver for `image` field that uses this additional data:
+
+
+```python
+from ariadne.asgi import GraphQL
+from ariadne_graphql_proxy import (
+    ProxySchema,
+    get_context_value,
+    set_resolver,
+)
+from graphql import build_ast_schema, parse
+
+
+proxy_schema = ProxySchema()
+
+# Store schema ID for remote schema
+remote_schema_id = proxy_schema.add_remote_schema(
+    "https://example.com/graphql/",
+)
+
+# Extend Product type with additional image field
+proxy_schema.add_schema(
+    build_ast_schema(
+        parse(
+            """
+            type Product {
+                image: String
+            }
+            """
+        )
+    )
+)
+
+# Configure proxy schema to retrieve thumb from metadata
+# from remote schema when image is queried
+proxy_schema.add_field_dependencies(
+    remote_schema_id, "Product", "image", "{ metadata { thumb } }"
+)
+
+# Create schema instance
+final_schema = proxy_schema.get_final_schema()
+
+
+# Add product image resolver
+def resolve_product_image(obj, info):
+    return obj["metadata"]["thumb"]
+
+
+set_resolver(final_schema, "Product", "image", resolve_product_image)
+
+
+# Setup Ariadne ASGI GraphQL application
+app = GraphQL(
+    final_schema,
+    context_value=get_context_value,
+    root_value=proxy_schema.root_resolver,
+    debug=True,
+)
+```
+
+
 ## Cache framework
 
 Ariadne GraphQL Proxy implements basic cache framework that enables of caching parts of GraphQL queries.
@@ -855,6 +920,34 @@ def add_delayed_fields(self, delayed_fields: Dict[str, List[str]]):
 Sets specific fields in schema as delayed. Delayed fields are excluded from queries ran by `root_resolver` against the remote GraphQL APIs.
 
 
+#### `delayed_fields`
+
+This is a dict of type name and fields names lists:
+
+```python
+{"Type": ["field", "otherField"], "OtherType": ["field"]}
+```
+
+
+### `add_field_dependencies`
+
+```python
+def add_field_dependencies(
+    self, schema_id: int, type_name: str, field_name: str, query: str
+):
+```
+
+Adds fields specified in `query` as dependencies for `field_name` of `type_name` that should be retrieved from schema with `schema_id`.
+
+
+#### Required arguments
+
+- `schema_id`: an `int` with ID of schema returned by `add_remote_schema` or `add_schema`.
+- `type_name`: a `str` with name of type for which dependencies will be set.
+- `field_name`: a `str` with name of field which dependencies will be set.
+- `query`: a `str` with additional fields to fetch when `field_name` is included, eg. `{ metadata { key value} }`.
+
+
 ### `add_foreign_key`
 
 ```python
diff --git a/ariadne_graphql_proxy/proxy_schema.py b/ariadne_graphql_proxy/proxy_schema.py
index bcb803c..587b273 100644
--- a/ariadne_graphql_proxy/proxy_schema.py
+++ b/ariadne_graphql_proxy/proxy_schema.py
@@ -227,10 +227,8 @@ def add_field_dependencies(
                 f"Type '{type_name}' in schema with ID '{schema_id}' is not "
                 "an object type."
             )
-        if field_name not in schema_type.fields:
-            raise ValueError(
-                f"Type '{type_name}' doesn't define the '{field_name}' field."
-            )
+
+        self.validate_field_with_dependencies(type_name, field_name)
 
         if schema_id not in self.dependencies:
             self.dependencies[schema_id] = {}
@@ -248,7 +246,14 @@ def add_field_dependencies(
             )
 
     def parse_field_dependencies(self, field_name: str, query: str) -> SelectionSetNode:
-        ast = parse(query)
+        clean_query = query.strip()
+        if not clean_query.startswith("{") or not clean_query.endswith("}"):
+            raise ValueError(
+                f"'{field_name}' field dependencies should be defined as a single "
+                "GraphQL operation, e.g.: '{ field other { subfield } }'."
+            )
+
+        ast = parse(clean_query)
 
         if (
             not len(ast.definitions) == 1
@@ -262,6 +267,19 @@ def parse_field_dependencies(self, field_name: str, query: str) -> SelectionSetN
 
         return ast.definitions[0].selection_set
 
+    def validate_field_with_dependencies(self, type_name: str, field_name: str) -> None:
+        for schema in self.schemas:
+            if (
+                type_name in schema.type_map
+                and isinstance(schema.type_map[type_name], GraphQLObjectType)
+                and field_name in schema.type_map[type_name].fields
+            ):
+                return
+
+        raise ValueError(
+            f"Type '{type_name}' doesn't define the '{field_name}' field in any of schemas."
+        )
+
     def add_delayed_fields(self, delayed_fields: Dict[str, List[str]]):
         for type_name, type_fields in delayed_fields.items():
             if type_name not in self.fields_map:
diff --git a/ariadne_graphql_proxy/selections.py b/ariadne_graphql_proxy/selections.py
new file mode 100644
index 0000000..875d7aa
--- /dev/null
+++ b/ariadne_graphql_proxy/selections.py
@@ -0,0 +1,53 @@
+from typing import Dict, Sequence, List, cast
+
+from graphql import FieldNode, SelectionNode, SelectionSetNode
+
+
+def merge_selection_sets(
+    set_a: SelectionSetNode, set_b: SelectionSetNode
+) -> SelectionSetNode:
+    return SelectionSetNode(
+        selections=tuple(merge_selections(set_a.selections, set_b.selections)),
+    )
+
+
+def merge_selections(
+    set_a: Sequence[SelectionNode], set_b: Sequence[SelectionNode]
+) -> List[SelectionNode]:
+    final_set: List[SelectionNode] = list(set_a)
+
+    index: Dict[str, int] = {}
+    for i, field in enumerate(final_set):
+        if isinstance(field, FieldNode):
+            index[(field.alias or field.name).value] = i
+
+    for field in set_b:
+        if isinstance(field, FieldNode):
+            field_name = (field.alias or field.name).value
+            if field_name in index:
+                field_index = index[field_name]
+                other_field = cast(FieldNode, final_set[field_index])
+                if other_field.selection_set and field.selection_set:
+                    final_set[field_index] = FieldNode(
+                        directives=other_field.directives,
+                        alias=other_field.alias,
+                        name=field.name,
+                        arguments=other_field.arguments,
+                        selection_set=merge_selection_sets(
+                            other_field.selection_set, field.selection_set
+                        ),
+                    )
+                elif other_field.selection_set or field.selection_set:
+                    final_set[field_index] = FieldNode(
+                        directives=other_field.directives,
+                        alias=other_field.alias,
+                        name=field.name,
+                        arguments=other_field.arguments,
+                        selection_set=(
+                            other_field.selection_set or field.selection_set
+                        ),
+                    )
+            else:
+                final_set.append(field)
+
+    return final_set
diff --git a/tests/test_merge_selection_sets.py b/tests/test_merge_selection_sets.py
new file mode 100644
index 0000000..c091cb4
--- /dev/null
+++ b/tests/test_merge_selection_sets.py
@@ -0,0 +1,82 @@
+from textwrap import dedent
+
+from graphql import parse, print_ast
+
+from ariadne_graphql_proxy import merge_selection_sets
+
+
+def test_merge_selection_sets_merges_two_flat_sets():
+    set_a = parse("{ hello }").definitions[0].selection_set
+    set_b = parse("{ world }").definitions[0].selection_set
+
+    result = merge_selection_sets(set_a, set_b)
+    assert (
+        print_ast(result)
+        == dedent(
+            """
+        {
+          hello
+          world
+        }
+        """
+        ).strip()
+    )
+
+
+def test_merge_selection_sets_merges_two_overlapping_flat_sets():
+    set_a = parse("{ hello world }").definitions[0].selection_set
+    set_b = parse("{ world }").definitions[0].selection_set
+
+    result = merge_selection_sets(set_a, set_b)
+    assert (
+        print_ast(result)
+        == dedent(
+            """
+        {
+          hello
+          world
+        }
+        """
+        ).strip()
+    )
+
+
+def test_merge_selection_sets_keeps_nested_selections():
+    set_a = parse("{ hello { sub } }").definitions[0].selection_set
+    set_b = parse("{ world }").definitions[0].selection_set
+
+    result = merge_selection_sets(set_a, set_b)
+    assert (
+        print_ast(result)
+        == dedent(
+            """
+        {
+          hello {
+            sub
+          }
+          world
+        }
+        """
+        ).strip()
+    )
+
+
+def test_merge_selection_sets_merges_selection_sets_recursively():
+    set_a = parse("{ hello { sub } }").definitions[0].selection_set
+    set_b = parse("{ hello { set } world }").definitions[0].selection_set
+
+    result = merge_selection_sets(set_a, set_b)
+    assert (
+        print_ast(result)
+        == dedent(
+            """
+        {
+          hello {
+            sub
+            set
+          }
+          world
+        }
+        """
+        ).strip()
+    )
diff --git a/tests/test_merge_selections.py b/tests/test_merge_selections.py
new file mode 100644
index 0000000..cfe6253
--- /dev/null
+++ b/tests/test_merge_selections.py
@@ -0,0 +1,82 @@
+from textwrap import dedent
+
+from graphql import SelectionSetNode, parse, print_ast
+
+from ariadne_graphql_proxy import merge_selections
+
+
+def test_merge_selections_merges_two_flat_sets():
+    set_a = parse("{ hello }").definitions[0].selection_set.selections
+    set_b = parse("{ world }").definitions[0].selection_set.selections
+
+    result = merge_selections(set_a, set_b)
+    assert (
+        print_ast(SelectionSetNode(selections=result))
+        == dedent(
+            """
+        {
+          hello
+          world
+        }
+        """
+        ).strip()
+    )
+
+
+def test_merge_selections_merges_two_overlapping_flat_sets():
+    set_a = parse("{ hello world }").definitions[0].selection_set.selections
+    set_b = parse("{ world }").definitions[0].selection_set.selections
+
+    result = merge_selections(set_a, set_b)
+    assert (
+        print_ast(SelectionSetNode(selections=result))
+        == dedent(
+            """
+        {
+          hello
+          world
+        }
+        """
+        ).strip()
+    )
+
+
+def test_merge_selections_keeps_nested_selections():
+    set_a = parse("{ hello { sub } }").definitions[0].selection_set.selections
+    set_b = parse("{ world }").definitions[0].selection_set.selections
+
+    result = merge_selections(set_a, set_b)
+    assert (
+        print_ast(SelectionSetNode(selections=result))
+        == dedent(
+            """
+        {
+          hello {
+            sub
+          }
+          world
+        }
+        """
+        ).strip()
+    )
+
+
+def test_merge_selections_merges_selection_sets_recursively():
+    set_a = parse("{ hello { sub } }").definitions[0].selection_set.selections
+    set_b = parse("{ hello { set } world }").definitions[0].selection_set.selections
+
+    result = merge_selections(set_a, set_b)
+    assert (
+        print_ast(SelectionSetNode(selections=result))
+        == dedent(
+            """
+        {
+          hello {
+            sub
+            set
+          }
+          world
+        }
+        """
+        ).strip()
+    )
diff --git a/tests/test_proxy_schema.py b/tests/test_proxy_schema.py
index 14d8a27..068b282 100644
--- a/tests/test_proxy_schema.py
+++ b/tests/test_proxy_schema.py
@@ -1265,6 +1265,36 @@ async def test_root_value_for_remote_schema_excludes_extensions(
     }
 
 
+@pytest.mark.asyncio
+async def test_add_field_dependencies_for_nonexisting_schema_raises_error(
+    httpx_mock, schema_json
+):
+    httpx_mock.add_response(json=schema_json)
+
+    proxy_schema = ProxySchema()
+    schema_id = proxy_schema.add_remote_schema("http://graphql.example.com/")
+
+    with pytest.raises(ValueError) as exc_info:
+        proxy_schema.add_field_dependencies(
+            schema_id + 1, "Complex", "invalid", "{ group { name } }"
+        )
+
+    assert "Schema with ID '1' doesn't exist." == str(exc_info.value)
+
+
+@pytest.mark.asyncio
+async def test_add_field_dependencies_for_local_schema_raises_error(schema):
+    proxy_schema = ProxySchema()
+    schema_id = proxy_schema.add_schema(schema)
+
+    with pytest.raises(ValueError) as exc_info:
+        proxy_schema.add_field_dependencies(
+            schema_id, "Complex", "invalid", "{ group { name } }"
+        )
+
+    assert "Schema with ID '0' is not a remote schema." == str(exc_info.value)
+
+
 @pytest.mark.asyncio
 async def test_add_field_dependencies_for_query_field_raises_error(
     httpx_mock, schema_json
@@ -1272,9 +1302,7 @@ async def test_add_field_dependencies_for_query_field_raises_error(
     httpx_mock.add_response(json=schema_json)
 
     proxy_schema = ProxySchema()
-    schema_id = proxy_schema.add_remote_schema(
-        "http://graphql.example.com/", proxy_extensions=False
-    )
+    schema_id = proxy_schema.add_remote_schema("http://graphql.example.com/")
 
     with pytest.raises(ValueError) as exc_info:
         proxy_schema.add_field_dependencies(schema_id, "Query", "basic", "{ complex }")
@@ -1291,9 +1319,7 @@ async def test_add_field_dependencies_for_mutation_field_raises_error(
     httpx_mock.add_response(json=schema_json)
 
     proxy_schema = ProxySchema()
-    schema_id = proxy_schema.add_remote_schema(
-        "http://graphql.example.com/", proxy_extensions=False
-    )
+    schema_id = proxy_schema.add_remote_schema("http://graphql.example.com/")
 
     with pytest.raises(ValueError) as exc_info:
         proxy_schema.add_field_dependencies(
@@ -1312,9 +1338,7 @@ async def test_add_field_dependencies_for_subscription_field_raises_error(
     httpx_mock.add_response(json=schema_json)
 
     proxy_schema = ProxySchema()
-    schema_id = proxy_schema.add_remote_schema(
-        "http://graphql.example.com/", proxy_extensions=False
-    )
+    schema_id = proxy_schema.add_remote_schema("http://graphql.example.com/")
 
     with pytest.raises(ValueError) as exc_info:
         proxy_schema.add_field_dependencies(
@@ -1333,9 +1357,7 @@ async def test_add_field_dependencies_for_nonexisting_type_raises_error(
     httpx_mock.add_response(json=schema_json)
 
     proxy_schema = ProxySchema()
-    schema_id = proxy_schema.add_remote_schema(
-        "http://graphql.example.com/", proxy_extensions=False
-    )
+    schema_id = proxy_schema.add_remote_schema("http://graphql.example.com/")
 
     with pytest.raises(ValueError) as exc_info:
         proxy_schema.add_field_dependencies(
@@ -1352,9 +1374,7 @@ async def test_add_field_dependencies_for_invalid_type_raises_error(
     httpx_mock.add_response(json=schema_json)
 
     proxy_schema = ProxySchema()
-    schema_id = proxy_schema.add_remote_schema(
-        "http://graphql.example.com/", proxy_extensions=False
-    )
+    schema_id = proxy_schema.add_remote_schema("http://graphql.example.com/")
 
     with pytest.raises(ValueError) as exc_info:
         proxy_schema.add_field_dependencies(
@@ -1373,16 +1393,57 @@ async def test_add_field_dependencies_for_nonexisting_type_field_raises_error(
     httpx_mock.add_response(json=schema_json)
 
     proxy_schema = ProxySchema()
-    schema_id = proxy_schema.add_remote_schema(
-        "http://graphql.example.com/", proxy_extensions=False
-    )
+    schema_id = proxy_schema.add_remote_schema("http://graphql.example.com/")
 
     with pytest.raises(ValueError) as exc_info:
         proxy_schema.add_field_dependencies(
             schema_id, "Complex", "invalid", "{ group { name } }"
         )
 
-    assert "Type 'Complex' doesn't define the 'invalid' field." == str(exc_info.value)
+    assert (
+        "Type 'Complex' doesn't define the 'invalid' field in any of schemas."
+        == str(exc_info.value)
+    )
+
+
+@pytest.mark.asyncio
+async def test_add_field_dependencies_with_invalid_dependencies_arg_raises_error(
+    httpx_mock, schema_json
+):
+    httpx_mock.add_response(url="http://graphql.example.com/", json=schema_json)
+
+    proxy_schema = ProxySchema()
+    schema_id = proxy_schema.add_remote_schema("http://graphql.example.com/")
+
+    with pytest.raises(ValueError) as exc_info:
+        proxy_schema.add_field_dependencies(
+            schema_id, "Complex", "class", "group { id }"
+        )
+
+    assert (
+        "'class' field dependencies should be defined as a single GraphQL "
+        "operation, e.g.: '{ field other { subfield } }'."
+    ) == str(exc_info.value)
+
+
+@pytest.mark.asyncio
+async def test_add_field_dependencies_with_invalid_dependencies_arg_op_raises_error(
+    httpx_mock, schema_json
+):
+    httpx_mock.add_response(url="http://graphql.example.com/", json=schema_json)
+
+    proxy_schema = ProxySchema()
+    schema_id = proxy_schema.add_remote_schema("http://graphql.example.com/")
+
+    with pytest.raises(ValueError) as exc_info:
+        proxy_schema.add_field_dependencies(
+            schema_id, "Complex", "class", "mutation { group { id } }"
+        )
+
+    assert (
+        "'class' field dependencies should be defined as a single GraphQL "
+        "operation, e.g.: '{ field other { subfield } }'."
+    ) == str(exc_info.value)
 
 
 @pytest.mark.asyncio