variables as parameters

[zmactep]

variables as parameters

[zmactep]

Hi, @Tshimanga!

The question is more about Cypher statement system then the driver itself. So, let us try to understand how this things works. There are no any string interpolation or something like this. Driver itself follow the protocol and do not provide any ORM or template system.

Each query is just a constant string, that is sent to the server. So we can run simple queries, that are presented by Text datatype, like "MATCH (n:Person {name: "Carl"}) RETURN n". So, this query will run just the same operation each time it is sent.

But of course we want some dynamic queries, where we can change values as we need:

• "MATCH (n:Person {name: "Carl"}) RETURN n"
• "MATCH (n:Person {name: "John"}) RETURN n"
• etc
  To make this possible we may try to construct a new Text request each time we send a request, like:

makeCypher :: String -> Text
makeCypher name = T.pack $ ""MATCH (n:Person {name: \"" ++ name ++ "\"}) RETURN n"

But this way is ugly in many cases. First, we have to construct this string and it is unpleasant itself. And second, neo4j server cannot cache such queries, as it see them as different statements (that is why usage of string interpolation is a bad idea here). To fix this problem we can use query parameters.

Neo4j lets us to send a statement and parameters separately. So it will cache our statements and just change some values in them. Parameters are presented as a map with string keys. So, we can place some {hole}s inside our statement and the name of the hole will we a key in such map. The system is pretty reach, as it let us use not only primitive types, but also maps and lists. Here is some examples:

let name = T "Carl"
let paramsMap = M (fromList [("name", name)])
queryP "MATCH (n:Person) WHERE n.name = {name} RETURN n" (fromList [("name", name)])
queryP "MATCH (n:Person {params}) RETURN n" (fromList [("params", paramsMap)])

I hope, it has clarified the situation a bit.

[Tshimanga]

Thanks @zmactep