Explain the behaviour of forEach

[NotJustAnna]

Apparently, forEach merge and sums all objects given to them? This is undocumented on the docs.

[chipotle]

What is it you're actually trying to do? forEach is described with:

Loop over a sequence, evaluating the given write query for each element.

And the examples you're giving are not write queries.

(I could have sworn that forEach had a warning somewhere that it was indeed only for write queries, but apparently not. But IIRC, it really means it, and I have a suspicion that what you want is more properly accomplished with map.)

[srh]

If you run something like

r.expr([1,2,3]).forEach(function(a) { return r.table('foo').insert({id: a}); })

it will output

{

    "deleted": 0 ,
    "errors": 0 ,
    "inserted": 3 ,
    "replaced": 0 ,
    "skipped": 0 ,
    "unchanged": 0
}

That result was formed by combining together three different write result objects. The .forEach function adds the stats together and concatenates any arrays together.

It looks like it doesn't validate that the query result is necessarily a write result, it just evaluates the subqueries. Hence

r.expr([1,2,3]).forEach(function(a) { return {x: a}; }) returning {x: 6}.

It will only complain if it gets a value it refuses to merge together, like a number.

I don't know why you're getting "var_541". I don't get that when I run the query. That's definitely a bug, really a complete mystery; what version of RethinkDB are you running it's clear from the screenshot you're running 2.4.

[srh]

It definitely could be better documented.

[srh]

Could I say again how freaky that var_541 thing is? I can't reproduce it in 2.4 or before. What browser are you running? Could it be its JavaScript engine?

[NotJustAnna]

Could I say again how freaky that var_541 thing is? I can't reproduce it in 2.4 or before. What browser are you running? Could it be its JavaScript engine?

It's the RethinkDB 2.4 Data Explorer

[NotJustAnna]

Managed to reproduce it with literal + instead of add

[srh]

That's because you wrote a + 1 instead of a.add(1). But in the ...

Okay. In the screenshot, I guess you ran a + 1, got the output, then edited the query to be a.add(1) without rerunning the query.

[NotJustAnna]

Just to get back on track:

That result was formed by combining together three different write result objects. The .forEach function adds the stats together and concatenates any arrays together.

It looks like it doesn't validate that the query result is necessarily a write result, it just evaluates the subqueries. Hence

r.expr([1,2,3]).forEach(function(a) { return {x: a}; }) returning {x: 6}.

It will only complain if it gets a value it refuses to merge together, like a number.

This is what's missing from the docs.

[NotJustAnna]

What is it you're actually trying to do? forEach is described with:

Loop over a sequence, evaluating the given write query for each element.

And the examples you're giving are not write queries.

(I could have sworn that forEach had a warning somewhere that it was indeed only for write queries, but apparently not. But IIRC, it really means it, and I have a suspicion that what you want is more properly accomplished with map.)

I'm not trying to do anything with it, I really tried to discover what the result type was. I have an issue tracking all RethinkDB types (https://github.com/rethinkdb/rethinkdb-java/issues/18), and forEach just returned an object. No format, explaining, nothing. Just object. That's when I stumbled upon this behaviour.