Array Operators
Array expansion
In order to access a named attribute from all elements in an array easily, AQL
offers the shortcut operator [*] for array variable expansion.
Using the [*] operator with an array variable will iterate over all elements
in the array, thus allowing to access a particular attribute of each element. It is
required that the expanded variable is an array. The result of the [*]
operator is again an array.
To demonstrate the array expansion operator, let’s go on with the following three example users documents:
[
{
"name": "john",
"age": 35,
"friends": [
{ "name": "tina", "age": 43 },
{ "name": "helga", "age": 52 },
{ "name": "alfred", "age": 34 }
]
},
{
"name": "yves",
"age": 24,
"friends": [
{ "name": "sergei", "age": 27 },
{ "name": "tiffany", "age": 25 }
]
},
{
"name": "sandra",
"age": 40,
"friends": [
{ "name": "bob", "age": 32 },
{ "name": "elena", "age": 48 }
]
}
]
With the [*] operator it becomes easy to query just the names of the
friends for each user:
FOR u IN users
RETURN { name: u.name, friends: u.friends[*].name }
This will produce:
[
{ "name" : "john", "friends" : [ "tina", "helga", "alfred" ] },
{ "name" : "yves", "friends" : [ "sergei", "tiffany" ] },
{ "name" : "sandra", "friends" : [ "bob", "elena" ] }
]
This is a shortcut for the longer, semantically equivalent query:
FOR u IN users
RETURN { name: u.name, friends: (FOR f IN u.friends RETURN f.name) }
Array contraction
In order to collapse (or flatten) results in nested arrays, AQL provides the [**]
operator. It works similar to the [*] operator, but additionally collapses nested
arrays.
How many levels are collapsed is determined by the amount of asterisk characters used.
[**] collapses one level of nesting - just like FLATTEN(array) or FLATTEN(array, 1)
would do -, [***] collapses two levels - the equivalent to FLATTEN(array, 2) - and
so on.
Let’s compare the array expansion operator with an array contraction operator. For example, the following query produces an array of friend names per user:
FOR u IN users
RETURN u.friends[*].name
As we have multiple users, the overall result is a nested array:
[
[
"tina",
"helga",
"alfred"
],
[
"sergei",
"tiffany"
],
[
"bob",
"elena"
]
]
If the goal is to get rid of the nested array, we can apply the [**] operator on the
result. But simply appending [**] to the query won’t help, because u.friends
is not a nested (multi-dimensional) array, but a simple (one-dimensional) array. Still,
the [**] can be used if it has access to a multi-dimensional nested result.
We can extend above query as follows and still create the same nested result:
RETURN (
FOR u IN users RETURN u.friends[*].name
)
By now appending the [**] operator at the end of the query…
RETURN (
FOR u IN users RETURN u.friends[*].name
)[**]
… the query result becomes:
[
[
"tina",
"helga",
"alfred",
"sergei",
"tiffany",
"bob",
"elena"
]
]
Note that the elements are not de-duplicated. For a flat array with only unique elements, a combination of UNIQUE() and FLATTEN() is advisable.
Inline expressions
It is possible to filter elements while iterating over an array, to limit the amount of returned elements and to create a projection using the current array element. Sorting is not supported by this shorthand form.
These inline expressions can follow array expansion and contraction operators
[* ...], [** ...] etc. The keywords FILTER, LIMIT and RETURN
must occur in this order if they are used in combination, and can only occur once:
anyArray[* FILTER conditions LIMIT skip,limit RETURN projection]
Example with nested numbers and array contraction:
LET arr = [ [ 1, 2 ], 3, [ 4, 5 ], 6 ]
RETURN arr[** FILTER CURRENT % 2 == 0]
All even numbers are returned in a flat array:
[
[ 2, 4, 6 ]
]
Complex example with multiple conditions, limit and projection:
FOR u IN users
RETURN {
name: u.name,
friends: u.friends[* FILTER CONTAINS(CURRENT.name, "a") AND CURRENT.age > 40
LIMIT 2
RETURN CONCAT(CURRENT.name, " is ", CURRENT.age)
]
}
No more than two computed strings based on friends with an a in their name and
older than 40 years are returned per user:
[
{
"name": "john",
"friends": [
"tina is 43",
"helga is 52"
]
},
{
"name": "sandra",
"friends": [
"elena is 48"
]
},
{
"name": "yves",
"friends": []
}
]
Inline filter
To return only the names of friends that have an age value
higher than the user herself, an inline FILTER can be used:
FOR u IN users
RETURN { name: u.name, friends: u.friends[* FILTER CURRENT.age > u.age].name }
The pseudo-variable CURRENT can be used to access the current array element.
The FILTER condition can refer to CURRENT or any variables valid in the
outer scope.
Inline limit
The number of elements returned can be restricted with LIMIT. It works the same
as the limit operation. LIMIT must come after FILTER
and before RETURN, if they are present.
FOR u IN users
RETURN { name: u.name, friends: u.friends[* LIMIT 1].name }
Above example returns one friend each:
[
{ "name": "john", "friends": [ "tina" ] },
{ "name": "sandra", "friends": [ "bob" ] },
{ "name": "yves", "friends": [ "sergei" ] }
]
A number of elements can also be skipped and up to n returned:
FOR u IN users
RETURN { name: u.name, friends: u.friends[* LIMIT 1,2].name }
The example query skips the first friend and returns two friends at most per user:
[
{ "name": "john", "friends": [ "helga", "alfred" ] },
{ "name": "sandra", "friends": [ "elena" ] },
{ "name": "yves", "friends": [ "tiffany" ] }
]
Inline projection
To return a projection of the current element, use RETURN. If a FILTER is
also present, RETURN must come later.
FOR u IN users
RETURN u.friends[* RETURN CONCAT(CURRENT.name, " is a friend of ", u.name)]
The above will return:
[
[
"tina is a friend of john",
"helga is a friend of john",
"alfred is a friend of john"
],
[
"sergei is a friend of yves",
"tiffany is a friend of yves"
],
[
"bob is a friend of sandra",
"elena is a friend of sandra"
]
]