mvellandi

Exploring Graphs with Elixir: Missing behaviour import, struct, cypher file (p.181)

Exploring Graphs with Elixir by @tonyhammond: P.181 — Ch 5 > Setting Up a Graph Service > API Demo

After starting iex in the umbrella project, then importing PropertyGraph, we are told to call:

iex> default_graph = read_graph("default.cypher")
 	#GraphCommons.Graph<type: property, file: "...", data: "CREATE (a)-[:EX]...">

However, read_graph hasn’t been defined yet.
We need to first add the following to the PropertyGraph module:

use GraphCommons.Graph, graph_type: :property, graph_module: __MODULE__

From that read_graph call, I don’t recall creating nor seeing the “default.cypher” in the source codebase:
graph_commons/priv/storage/property/graphs/default.cypher

I believe perhaps a cypher file with the following should work:

CREATE (a)-[:EX]->(b)

Shortly later, we call:

iex> graph_info
%GraphCommons.Service.GraphInfo{
 	  labels: [],
 	  num_edges: 1,
 	  num_nodes: 2,
 	  type: :property
}

For me, this returned an undefined function error for PropertyGraph.new_query/1
So I looked at the definition of graph_info/0 in the book’s previous section on p.182

def graph_info() do
 	  {:ok, [stats]} =
 	    @cypher_info
 	    |> PropertyGraph.new_query()
 	    |> query_graph
 	
 	  %GraphCommons.Service.GraphInfo{
 	    type: :property,
 	    num_nodes: stats["nodeCount"],
 	    num_edges: stats["relCount"],
 	    labels: Map.keys(stats["labels"])
 	  }
 end

I noticed
1:
Although GraphCommons.Query hasn’t been formally defined, by duplicating GraphCommons.Graph to GraphCommons.Query
and changing “graph” to “query” references and checking the module’s source codebase, we should have a new_query/3 function.

However, PropertyGraph.new_query is called here with only 2 out of 3 arguments: @cypher_info and (implicitly) the query_type
Is there a missing argument for query_file ?
I couldn’t figure this out.

2:
And for graph_info/0 to return a %GraphCommons.Service.GraphInfo{} struct, I believe we need to first define the module which I didn’t see in the source codebase. So I made one here:
graph_commons/lib/graph_commons/service/graph_info.ex

Searching the source codebase for GraphInfo structs, I found the following attributes variously used, and so defined the module as follows:

defmodule GraphCommons.Service.GraphInfo do
    defstruct ~w[type num_nodes num_edges labels density file]a
end

1 comment

#errata /book-exploring-graphs-with-elixir

0 835 1

2021-05-03 21:50:30 UTC

First Post!

mvellandi

Regarding #1 above, getting PropertyGraph.new_query to work with one or two arguments:
I revised the definition of “using” macro in GraphCommons.Query to use a default arg:

def new_query(query_data, query_file \\ "") do
   if unquote(query_type) in [:dgraph, :native, :property, :rdf, :tinker] do
      GraphCommons.Query.new(query_data, query_file, unquote(query_type))
   end
end

Simple fix.
A code editor’s language server may need some time to recognize this as not an issue.

If you now run PropertyGraph.graph_info/0 again and get this next error below (like I did):

{ :error,
   %Bolt.Sips.Error{
      code: "Neo.ClientError.Procedure.ProcedureNotFound", 
      message: "There is no procedure with the name `apoc.meta.stats` registered for this database instance. Please ensure you've spelled the procedure name correctly and that the procedure is properly deployed."
   }
}

It means APOC-Core, included with Neo4j, hasn’t been added as a plugin.
Lastly, the Neo4j security settings configuration may need a one-line config for enabling plugin/other resource full database access.
I’ve written another post on DevTalk with how I installed Neo4j on OSX.

Cheers

Post #2