Using c# to connect To Neo4j (Graph database) Part 2

13 במרץ 2013

25 תגובות

In the first part I have talked about graph database in general and Neo4j in particular. I have covered the installation of Neo4j and the different ways to connected to it as a c# developer. We talked about the key elements in graph database: nodes and relationships, and how to create them in Neo4j graph database. If you haven't read the first part click here before you continue.

In order to use the full functionality of Neo4j you will need to learn to use cypher query language, it is quite simple and most of the code I will write in this post will use it.
Here is a small and helpful method I have created to execute cypher query on the Neo4j database and return the results from it:

private async Task<string> CypherQueryRunner(string query)
{
    string retVal = null;

    HttpClient httpClient = CreateHttpClient();
    HttpContent stringContent = new StringContent(query);

    HttpResponseMessage av = await httpClient.PostAsync
        ("db/data/cypher", stringContent);

    if (av.IsSuccessStatusCode)
    {
        retVal = await av.Content.ReadAsStringAsync();
    }

    return retVal;
}

This method is very simple, it gets a string contains cypher query and execute it on the Neo4j server, and return the results as string from the Neo4j server.

At the first post we learned how to create a node and how to create a relationship in Neo4j graph database, now I would like to retrieve data from my database and convert it back my entities and really use the power of the graph database.

Here is a method that reads entities from the database, after the code I will explain important elements in the method:

1. public async Task<List<Entity>> Read(List<int> nodeIds, 
2.    List<string> filterByLinkTypes , int depth = 1)
3. {
4.    List<Entity> retVal = null;
5.    
6.    var nodeIdsStr = BuildNodeIDString(nodeIds);
7.    var relationshipTypesFilterStr =  
8.       buildRelationshipTypesFilterStr(filterByLinkTypes);
9.    if (depth == 1)
10.    {
11.        var readQuery = "{  \"query\" : \"start n = node(" + 
12.             nodeIdsStr + ") return n\",  \"params\" : {  }}";
13.        var response = await CypherQueryRunner(readQuery);
14.
15.        if (response != null)
16.        {
17.            retVal = CreateEntitiesFromDatabase(
18.              JsonConvert.DeserializeObject<Neo4jResults>(response));
19.        }
20.    }
21.    else
22.    {
23.        var readQuery = "{  \"query\" : \"start n = node(" + nodeIdsStr 
24.      +") match n-["+relationshipTypesFilterStr+"*1.." + depth + "]-m"
25.      +" WHERE m.NodeType!='Entity' return ID(m) \",\"params\" : { }}";
26.
27.        var response = await CypherQueryRunner(readQuery);
28.        if (response != null)
29.        {
30.            var nodeIdsRespond = JsonConvert.DeserializeObject
31.                    <NodeIdsRespond>(response);
32.            nodeIds = GetNodeIdsFromNodeIdsRespond(nodeIdsRespond)
33.                    .Union(nodeIds).ToList<int>();
34.            retVal = await Read(nodeIds, filterByLinkTypes, 1);
35.        }
36.    }
37.
38.    return retVal;
39. }

Explanation:

Lines 1 and 2 – the method gets a list of database node ids for retrieving, a list of string that represents the relationship types (family, friends…) and depth that represent whether I like to retrieve just the nodes on in the nodes id list or also the nodes connected to them until certain depth.

Line 6 – Build a string with the node ids (e.g. 1,2,7,8)

Lines 7 and 8 – Build a string of relationship types (e.g. friend, employ, x-girlfriend)

Lines 10-20 – this case we are handling the case of depth 1, meaning need to retrieve only the entities with ids in the input nodes id list – simple case, run an cypher query and deserialized back to objects.

Lines 23-25 In case the we need to retrieve entities with ids in the input nodes id list AND entities that have relationships to them (filtered by the input filterByLinkTypes), in this cypher query I am searching for those database node that fulfill the depth and relationship types according to the input from the initial node ids.

Line 32 and line 33 – get the results node ids from the Neo4j respond and union them with the input node ids list.

Line 34 – call this method again (yes recursion), combine nodeIds, initial filter types but now with the depth 1.

As you can see, of course we could have implemented this on a relational databases, but think what would happened if the depth will be 5, or 10 or even larger than 10. The performance in relational databases will be much (much!) bigger than the performance on a graph database and for those queries graph databases excel and this is the real power of graph databases.

What if you would like to update a node ? Very very easy using cypher:

public async Task Update(int nodeIndex, 
        Dictionary<string, string> keysAndValues)
{
    // A method that build the properties string 
    //(e.g. n.Name = 'Someone', n.Age = 35
    var updateStr = BuildUpdateString(keysAndValues);
    string updateQuery = "{  \"query\" : \"start n = node(" + 
        nodeIndex + ") SET " + updateStr + "  \",  \"params\" : {  }}";
    await CypherQueryRunner(updateQuery);
}

This method gets a database node index and a dictionary that represents the new properties of the node and using a simple cypher query update the node in the database.

And last operation in this example: delete, again very very simple let's look:

public async Task Delete(List<Entity> entities)
{
    // Build a string of node ids (e.g. 1,3,7)
    var NodeIDStr = BuildNodeIDString(entities);
    string updateQuery = "{  \"query\" : \"start n = node(" + NodeIDStr + 
        ") MATCH n-[r?]-() DELETE n, r;  \",  \"params\" : {  }}";
    await CypherQueryRunner(updateQuery);
}

This method get a list on database node ids and delete them from the database including all the relationship that they are a part of.

I hope you enjoyed and learned a bit about graph databases in general and Neo4j in particular. In those two blog posts I showed all the CRUD operations on neo4j database and how to work with cypher query language. There are more to learn but I Hope you realized the basics and understand the true power of a graph database.


Until next time, be happy.

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