add support for sub-graph querying in api

backend/lib/backend/api.ex

@@ -21,9 +21,11 @@ defmodule Backend.Api do
   * have a user count (required to give the instance a size on the graph)
   * the user count is > the threshold
   * have x and y coordinates
+
+  If `domain` is passed, then this function only returns nodes that are neighbors of that instance.
   """
   @spec list_nodes() :: [Instance.t()]
-  def list_nodes() do
+  def list_nodes(domain \\ nil) do
     user_threshold = get_config(:personal_instance_threshold)
 
     Instance
@@ -32,17 +34,37 @@ defmodule Backend.Api do
       not is_nil(i.x) and not is_nil(i.y) and not is_nil(i.user_count) and
         i.user_count >= ^user_threshold
     )
+    |> maybe_filter_nodes_to_neighborhood(domain)
     |> select([c], [:domain, :user_count, :x, :y])
     |> Repo.all()
   end
 
+  # if we're getting the sub-graph around a given domain, only return neighbors.
+  defp maybe_filter_nodes_to_neighborhood(query, domain) do
+    case domain do
+      nil ->
+        query
+
+      _ ->
+        query
+        |> join(:inner, [i], outgoing_edges in Edge, on: outgoing_edges.source_domain == i.domain)
+        |> join(:inner, [i], incoming_edges in Edge, on: incoming_edges.target_domain == i.domain)
+        |> where(
+          [i, outgoing_edges, incoming_edges],
+          outgoing_edges.target_domain == ^domain or incoming_edges.source_domain == ^domain
+        )
+        |> distinct(true)
+    end
+  end
+
   @spec list_edges() :: [Edge.t()]
-  def list_edges() do
+  def list_edges(domain \\ nil) do
     user_threshold = get_config(:personal_instance_threshold)
 
     Edge
     |> join(:inner, [e], i1 in Instance, on: e.source_domain == i1.domain)
     |> join(:inner, [e], i2 in Instance, on: e.target_domain == i2.domain)
+    |> maybe_filter_edges_to_neighborhood(domain)
     |> select([e], [:id, :source_domain, :target_domain, :weight])
     |> where(
       [e, i1, i2],
@@ -53,6 +75,27 @@ defmodule Backend.Api do
     |> Repo.all()
   end
 
+  defp maybe_filter_edges_to_neighborhood(query, domain) do
+    case domain do
+      nil ->
+        query
+
+      _ ->
+        # we want all edges in the neighborhood -- not just edges connected to `domain`
+        query
+        |> join(:inner, [e], neighbor_edges in Edge,
+          on:
+            neighbor_edges.source_domain == e.target_domain or
+              neighbor_edges.target_domain == e.source_domain
+        )
+        |> where(
+          [e, i1, i2, neighbor_edges],
+          e.source_domain == ^domain or e.target_domain == ^domain or
+            neighbor_edges.source_domain == ^domain or neighbor_edges.target_domain == ^domain
+        )
+    end
+  end
+
   def search_instances(query, cursor_after \\ nil) do
     ilike_query = "%#{query}%"
 

backend/lib/backend_web/controllers/graph_controller.ex

@@ -10,4 +10,10 @@ defmodule BackendWeb.GraphController do
     edges = Api.list_edges()
     render(conn, "index.json", nodes: nodes, edges: edges)
   end
+
+  def show(conn, %{"id" => domain}) do
+    nodes = Api.list_nodes(domain)
+    edges = Api.list_edges(domain)
+    render(conn, "index.json", nodes: nodes, edges: edges)
+  end
 end

backend/lib/backend_web/router.ex

@@ -9,7 +9,7 @@ defmodule BackendWeb.Router do
     pipe_through(:api)
 
     resources("/instances", InstanceController, only: [:index, :show])
-    resources("/graph", GraphController, only: [:index])
+    resources("/graph", GraphController, only: [:index, :show])
     resources("/search", SearchController, only: [:index])
   end
 end