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Neo4j 2.0.1 & Cypher: Concatenating an Empty Collection

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Neo4j 2.0.1 & Cypher: Concatenating an Empty Collection

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Last weekend I was playing around with some collections using Neo4j’s Cypher query language and I wanted to concatenate two collections.

This was easy enough when both collections contained values…

$ RETURN [1,2,3,4] + [5,6,7];
==> +---------------------+
==> | [1,2,3,4] + [5,6,7] |
==> +---------------------+
==> | [1,2,3,4,5,6,7]     |
==> +---------------------+
==> 1 row

…but I ended up with the following exception when I tried to concatenate with an empty collection:

$ RETURN [1,2,3,4] + [];
==> SyntaxException: Type mismatch: expected Integer, Collection<Integer> or Collection<Collection<Integer>> but was Collection<Any> (line 1, column 20)
==> "RETURN [1,2,3,4] + []"
==>                     ^

I figured there was probably some strange type coercion going on for the empty collection and came up with the following work around using the RANGE function:

$ RETURN [1,2,3,4] + RANGE(0,-1);
==> +-------------------------+
==> | [1,2,3,4] + RANGE(0,-1) |
==> +-------------------------+
==> | [1,2,3,4]               |
==> +-------------------------+
==> 1 row

While writing this up I decided to check if it behaved the same way in the recently released 2.0.2 and was pleasantly surprised to see that the work around is no longer necessary:

$ RETURN [1,2,3,4] + [];
==> +----------------+
==> | [1,2,3,4] + [] |
==> +----------------+
==> | [1,2,3,4]      |
==> +----------------+
==> 1 row

So if you’re seeing the same issue get yourself upgraded!

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