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Mark Needham
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Thoughts on Software Development
Updated: 4 hours 26 min ago

Neo4j: Cypher – UNWIND vs FOREACH

Sat, 05/31/2014 - 16:19

I’ve written a couple of posts about the new UNWIND clause in Neo4j’s cypher query language but I forgot about my favourite use of UNWIND, which is to get rid of some uses of FOREACH from our queries.

Let’s say we’ve created a timetree up front and now have a series of events coming in that we want to create in the database and attach to the appropriate part of the timetree.

Before UNWIND existed we might try to write the following query using FOREACH:

WITH [{name: "Event 1", timetree: {day: 1, month: 1, year: 2014}}, 
      {name: "Event 2", timetree: {day: 2, month: 1, year: 2014}}] AS events
FOREACH (event IN events | 
  CREATE (e:Event {name: event.name})
  MATCH (year:Year {year: event.timetree.year }), 
        (year)-[:HAS_MONTH]->(month {month: event.timetree.month }),
        (month)-[:HAS_DAY]->(day {day: event.timetree.day })
  CREATE (e)-[:HAPPENED_ON]->(day))

Unfortunately we can’t use MATCH inside a FOREACH statement so we’ll get the following error:

Invalid use of MATCH inside FOREACH (line 5, column 3)
"  MATCH (year:Year {year: event.timetree.year }), "
   ^
Neo.ClientError.Statement.InvalidSyntax

We can work around this by using MERGE instead in the knowledge that it’s never going to create anything because the timetree already exists:

WITH [{name: "Event 1", timetree: {day: 1, month: 1, year: 2014}}, 
      {name: "Event 2", timetree: {day: 2, month: 1, year: 2014}}] AS events
FOREACH (event IN events | 
  CREATE (e:Event {name: event.name})
  MERGE (year:Year {year: event.timetree.year })
  MERGE (year)-[:HAS_MONTH]->(month {month: event.timetree.month })
  MERGE (month)-[:HAS_DAY]->(day {day: event.timetree.day })
  CREATE (e)-[:HAPPENED_ON]->(day))

If we replace the FOREACH with UNWIND we’d get the following:

WITH [{name: "Event 1", timetree: {day: 1, month: 1, year: 2014}}, 
      {name: "Event 2", timetree: {day: 2, month: 1, year: 2014}}] AS events
UNWIND events AS event
CREATE (e:Event {name: event.name})
WITH e, event.timetree AS timetree
MATCH (year:Year {year: timetree.year }), 
      (year)-[:HAS_MONTH]->(month {month: timetree.month }),
      (month)-[:HAS_DAY]->(day {day: timetree.day })
CREATE (e)-[:HAPPENED_ON]->(day)

Although the lines of code has slightly increased the query is now correct and we won’t accidentally correct new parts of our time tree.

We could also pass on the event that we created to the next part of the query which wouldn’t be the case when using FOREACH.

Categories: Blogs

Neo4j: Cypher – Neo.ClientError.Statement.ParameterMissing and neo4j-shell

Sat, 05/31/2014 - 14:44

Every now and then I get sent Neo4j cypher queries to look at and more often than not they’re parameterised which means you can’t easily run them in the Neo4j browser.

For example let’s say we have a database which has a user called ‘Mark’:

CREATE (u:User {name: "Mark"})

Now we write a query to find ‘Mark’ with the name parameterised so we can easily search for a different user in future:

MATCH (u:User {name: {name}}) RETURN u

If we run that query in the Neo4j browser we’ll get this error:

Expected a parameter named name
Neo.ClientError.Statement.ParameterMissing

If we try that in neo4j-shell we’ll get the same exception to start with:

$ MATCH (u:User {name: {name}}) RETURN u;
ParameterNotFoundException: Expected a parameter named name

However, as Michael pointed out to me, the neat thing about neo4j-shell is that we can define parameters by using the export command:

$ export name="Mark"
$ MATCH (u:User {name: {name}}) RETURN u;
+-------------------------+
| u                       |
+-------------------------+
| Node[1923]{name:"Mark"} |
+-------------------------+
1 row

export is a bit sensitive to spaces so it’s best to keep them to a minimum. e.g. the following tries to create the variable ‘name ‘ which is invalid:

$ export name = "Mark"
name  is no valid variable name. May only contain alphanumeric characters and underscores.

The variables we create in the shell don’t have to only be primitives. We can create maps too:

$ export params={ name: "Mark" }
$ MATCH (u:User {name: {params}.name}) RETURN u;
+-------------------------+
| u                       |
+-------------------------+
| Node[1923]{name:"Mark"} |
+-------------------------+
1 row

A simple tip but one that saves me from having to rewrite queries all the time!

Categories: Blogs

Clojure: Destructuring group-by’s output

Sat, 05/31/2014 - 02:03

One of my favourite features of Clojure is that it allows you to destructure a data structure into values that are a bit easier to work with.

I often find myself referring to Jay Fields’ article which contains several examples showing the syntax and is a good starting point.

One recent use of destructuring I had was where I was working with a vector containing events like this:

user> (def events [{:name "e1" :timestamp 123} {:name "e2" :timestamp 456} {:name "e3" :timestamp 789}])

I wanted to split the events in two – those containing events with a timestamp greater than 123 and those less than or equal to 123.

After remembering that the function I wanted was group-by and not partition-by (I always make that mistake!) I had the following:

user> (group-by #(> (->> % :timestamp) 123) events)
{false [{:name "e1", :timestamp 123}], true [{:name "e2", :timestamp 456} {:name "e3", :timestamp 789}]}

I wanted to get 2 vectors that I could pass to the web page and this is fairly easy with destructuring:

user> (let [{upcoming true past false} (group-by #(> (->> % :timestamp) 123) events)] 
       (println upcoming) (println past))
[{:name e2, :timestamp 456} {:name e3, :timestamp 789}]
[{:name e1, :timestamp 123}]
nil

Simple!

Categories: Blogs