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Table of contents

Filtering and Boosting

Filtering

In filtering, the expression must return boolean value for each item. If the returned value is true, the item passed the filter; if the value is false, the item does not pass and will be discarded. The value is computed from set property values of the individual items.

Consider the following table of items in sample movie-recommendation domain:

Name
string
Year
int
Director
string
Genres
set
Parental-Advisory
boolean
Pulp Fiction1994Quentin Tarantino{"Crime Fiction","Drama", "Thriller" }true
King Kong2005Peter Jackson{"Action", "Drama", "Adventure" }false
Fight Club1999David Fincher{"Drama", "Existentialism"}true
The Lord of the Rings: The Return of the King2003Peter Jackson{ "Adventure", "Fantasy", "Action" }false
The Dark Knight2008Christopher Nolan{"Superhero","Drama" ,"Action", "Adventure", "Thriller", "Crime Fiction"}false
Silence of the Lambs1991Jonathan Demme{"Crime Fiction", "Drama", "Thriller", "Horror" }true
Dead Alive1992Peter Jackson{"Horror", "Comedy"}true
… and 10000 other movies

Example 1

By default, when items are to be recommended to a given user, the recommender selects any items which seem relevant to the user. However, it may be your policy not to recommend items with Parental-Advisory flag set on. Hence you may use the following simple ReQL filtering expression:

ReQL
not 'Parental-Advisory'

Then the recommender may only choose from the following movies:

Name
string
Year
int
Director
string
Genres
set
Parental-Advisory
boolean
King Kong2005Peter Jackson{"Action", "Drama", "Adventure" }false
The Lord of the Rings: The Return of the King2003Peter Jackson{ "Adventure", "Fantasy", "Action" }false
The Dark Knight2008Christopher Nolan{"Superhero","Drama" ,"Action", "Adventure", "Thriller", "Crime Fiction"}false
… and 5926 other movies

Example 2

If you want to allow only items without Parental-Advisory which were directed by Peter Jackson (for example because a user selected such a filter at your site) you can do it by:

ReQL
(not 'Parental-Advisory') and ('Director' == "Peter Jackson")
note20
Note

You can access value of a property by putting name of the property into the single quotes. Strings are enclosed in double quotes.

Only following items can be recommended:

Name
string
Year
int
Director
string
Genres
set
Parental-Advisory
boolean
King Kong2005Peter Jackson{"Action", "Drama", "Adventure" }false
The Lord of the Rings: The Return of the King2003Peter Jackson{ "Adventure", "Fantasy", "Action" }false

Example 3

As another example, consider that user entered the Thriller section of your system’s catalog. Then you sure wish to recommend thrillers, ignoring the fact that usually, the user likes comedies.

As the Genres is a set property, you can use the in operator for checking whether Thriller is listed in the item’s genres:

ReQL
"Thriller" in 'Genres'

Only the following items pass the filter:

Name
string
Year
int
Director
string
Genres
set
Parental-Advisory
boolean
Pulp Fiction1994Quentin Tarantino{"Crime Fiction","Drama", "Thriller" }true
The Dark Knight2008Christopher Nolan{"Superhero","Drama" ,"Action", "Adventure", "Thriller", "Crime Fiction"}false
Silence of the Lambs1991Jonathan Demme{"Crime Fiction", "Drama", "Thriller", "Horror" }true
… and 3141 other movies

Handling deleted items

Filtering offers you an elegant way of handling deleted/obsolete items in the catalog. In many situations, it may happen that some items become unavailable and hence should not be recommended anymore. Considering interaction data, however, such items may still be important for the recommender. For example, the recommender may find out that users who liked a no more available item, x, will probably like another item, y, which is still available. Therefore, it is undesirable to simply delete x, deleting also all the related interactions in cascade.

With filtering, you may handle item deletes using the following scheme:

  • Create a dedicated item property, such as deleted, of type boolean (the implicit value for all items will be null, which is OK).

  • For deleted items, set the value of deleted true.

  • For recommendations, use the following filter:

    ReQL
    not 'deleted'

  • If the item becomes available again, you may set deleted to false.

Such a mechanism cay easily be extended to control availability over different regions, customer licenses, etc.

Boosting

In advanced applications, besides filtering, you may wish to boost recommendation rates of some items. In contrast to filtering, where items may be completely blocked, in boosting, you may tell the recommender to prefer some items among others. Indeed, by default, it is a task of the recommender itself to select the items which are the most relevant. However, it may be your policy to purposefully bias the recommender toward your business goals.

For example, considering the above table of movies, one may wish to promote the movies which are new and were filmed after 2000, especially if they were filmed after 2005. Then the following boosting query can handle that:

ReQL
if 'Year' <= 2000 then 1 else (if 'Year' <= 2005 then 1.5 else 2)

As you can see, boosting expressions return numbers rather than booleans as in case of filtering. Specifically, they provide the items with coefficients by which the internal scores determined by the recommender will be multiplied.

The boosting coefficients assigned by the query are shown in the following table:

Name
string
Year
int
Boosting
Pulp Fiction19941.0
King Kong20051.5
Fight Club19991.0
The Lord of the Rings: The Return of the King20031.5
The Dark Knight20082.0
Silence of the Lambs19911.0
Dead Alive19921.0
… and 10000 other movies

Examples

Exclude some items from recommendations using their IDs

Use filter:

ReQL
'itemId' not in {"item-127", "item-756", "item-568"}

The three items will not be recommended.

Re-rank a given set of items

Use filter (usually sent as a dynamic parameter of the API recommendation request):

ReQL
'itemId' in {"item-42", "item-77", "item-1992"}
  • The recommendation engine will re-rank the given set of items for the particular user.
  • It is supported to provide thousands of IDs in the query

Suppose that the items have a string property manufacturer and the user is viewing a product detail page.

If you want to give the user related items using Recommend Items to Item, but restrict them to the manufacturer of the currently viewed item, use filter:

ReQL
'manufacturer' == context_item["manufacturer"]

context_item function is used for retrieving property values of the item, that is currently viewed by the user.

Boost items that were published in last 24 hours

Suppose that the items have a timestamp property published_date. Then you can use this booster:

ReQL
if 'published_date' >= now() - 24 * 60 * 60 then 2 else 1

now returns current UTC timestamp in seconds.

24 * 60 * 60 is the number of seconds in 24 hours.

Up-sell

Suppose that the items have a double property price. Slightly boost items that are more expensive then the currently viewed one with following booster in Recommend Items to Item:

ReQL
if 'price' > context_item["price"] then 1.2 else 1

Recommend only items available in user’s city

  • Suppose that the items have a set property cities. It contains cities in which the items are available.
  • Suppose that the users have a string property city giving the city where each user live.

To recommend only items available in a user’s city, use filter:

ReQL
context_user["city"] in 'cities'

context_user function is used for retrieving property values of the user for which you request the recommendations.

Recommend only items from subscribed topics

Assume your site contains articles about various topics. The users can choose which topics are interesting for them in order to receive personalized newsletters with articles from these topics.

The subscribed topics are stored for each user in a User property of type set called subscribed_topics.

Each article can belong to multiple topics, stored in the topics item property of the type set.

The following filter allows only items that contain at least one matching topic to be recommended:

ReQL
size(context_user["subscribed_topics"] & 'topics') > 0

The & operator returns the intersection of the two sets, which must be non-empty.

Value Types and Operators

ReQL support following value types:

  • int – signed integer (currently 64bit),
  • double – double-precision floating-point number (IEEE 754 compliant),
  • timestamp – UTC timestamp, similar to double,
  • string – sequence of Unicode characters,
  • boolean – binary data type of two possible values: true or false,
  • set – unordered collection of values.
  • array – ordered collection of values.

Except for set and array types, all of the types include special value of null, which, again, corresponds to the fact that null is an allowed and also default value for the item / user property values in the API.

Numbers

Notation

ExpressionEquivalentComment
0123.000123.0

Leading and trailing zeros are ignored.

1.23e+31230.0

Exponential notation may be used.

1e91000000000

Using simple exponential notation for huge numbers.

123E-21.23

Negative exponents may also be used. Case of the e character does not matter.

Operations

ExpressionResultComment
1 + 23

Addition.

1 + 2 + 3 + 410

Chain of additions.

1 - 2-1

Subtraction.

1 - 2 - 3 - 4-9

Chain of subtractions.

-(1 + 2)-3

Unary minus.

2 * 36

Multiplication.

1 + 2 * 3 - 43

Standard operator precedence.

(1 + 2) * (3 - (4 + 5))-18

Bracketing.

10 / 52.0

Division.

1 / 20.5

Division always results in double, event if the operands are integers!

5 / 0NaN

If the divisor is 0, the result is NaN.

9 % 41

Modulo division.

3.14 % 2.50.64

Modulo division also works for doubles.

5 % 0NaN

If the divisor is 0, the result is NaN.

Comparison

ExpressionResultComment
1 < 2.0true

Integers, doubles, and timestamps may be compared using standard comparison operators.

1 < 2 <= 2 == 2 != 1 >= 1 > 0true

Comparison operators may be arbitrarily chained.

1 < 2 <= 2 == 3 != 1 >= 1 > 0false

Chain of comparisons returns true if and only if all the individual comparisons are true.

2 == 2.0true

In comparison, there is no difference between integers, doubles, and timestamps.

Strings

Notation

ExpressionComment
"foo"

Strings constants are enclosed in double quotes.

""

Empty string.

"she said \"hello\""

Double quotes must be escaped.

"she said 'hello'"

Single quotes needn’t be escaped.

Comparison

ExpressionResultComment
"foo" == "foo"true

Strings are compared for equality with ==.

"Alice" != nulltrue

Strings can be compared to null.

"Alice" < "Bob"true

Strings are ordered in lexicographic order.

"Alice" < "Bob" < "Carol" < "Dan"true

Comparisons may be chained arbitrarily.

"Alice" < "Bob" <= "Carol" != "Dan"true

Comparisons in the chain may be of different types.

"Alice" < "Bob" >= "Carol" != "Dan"false

All the comparisons must hold for the chain to return true.

"Alice" < 5error

Strings are only comparable with strings.

"Alice" ~ "A[a-z]+"true

Strings can be matched with regular expressions (regex).

Containment

ExpressionResultComment
"ice" in "Alice"true

in operator between strings tests whether the first string is contained in the second string.

"Ice" in "Alice"false

Containment test is case sensitive.

"ice" not in "Alice"false

in operator may be negated for better readability.

"" in "abc"true

Empty string is contained in every string.

"abc" in ""false

No non-empty string is contained in empty string.

5 in "abc"error

Both operands must be strings for string containment testing.

Concatenation

ExpressionResultComment
"foo" + "bar""foobar"

Strings can be concatenated using the + operator.

"" + "foo" + """foo"

Empty string is neutral element for concatenation.

"foo" + 123"foo123"

Strings can be concatenated with integers.

"foo" + 123.0"foo123.0"

Strings can be concatenated with numbers.

Indexing and Slicing

ExpressionResultComment
"abcd"[1]"b"

A character in the string can be accessed by its index (starting from 0).

"abcd"[10]""

Requesting an index outside the array boundaries results in an empty string.

"abcd"[-1]"d"

Negative indices are interpreted as counting from the end of the string. -1 therefore means the last character.

"abcd"[1:3]"bc"

It is possible to get a sub-string between two indices.

"abcd"[1:]"bcd"

If the second index is omitted, all characters until the end of the string are returned.

Sets

Notation

ExpressionComment
{}

Empty set.

{1, 2, 3}

Set containing three integers.

{1, 2.0, false, "foo", null}

Sets may contain values of different types. This is an extension to sets in the API, which may only contain strings.

{{1,2}, {2,3}}

Sets may be nested.

Properties

ExpressionResultComment
{ 1, 1, 1, 2 }{ 1, 2 }

Sets only contain unique elements.

{ 1, 1.0 }{ 1.0 }

Integers, doubles, and timestamps, are merged.

{ {1,2}, {2,1} }{ {1,2} }

Merging also works for nested sets.

Value Containment

ExpressionResultComment
2 in { 1, 2, 3 }true

Using in operator, you may test whether a value is contained in given set (the ∈ relation)

4 not in { 1, 2, 3 }true

The in operator may be negated for better readability (the ∉ relation).

2.0 in { 1, 2, 3 }true

There is no difference between integers, doubles, and timestamps when testing containment.

"2" in { 1, 2, 3 }false

There is a difference between numbers and strings.

{ 1, 2 } in { 1, 2, 3 }false

in stands for ∈, not ⊆!

{ 1, 2 } in { {1,2}, {3,4} }true

in stands for ∈.

Comparison

ExpressionResultComment
{ 1, 2 } < { 1, 2, 3 }true

Using < operator, you may test whether one test is a proper subset of another set (⊂ operator in set algebra).

{ 1, 2 } < { 1, 2 }false

No set is a proper subset of itself.

{} < { 1, 2 }true

Empty set is a proper subset of every non-empty set.

{} < {}false

Empty set is not a proper subset of itself.

{ 1, 2 } <= { 1, 2, 3 }true

Using <= operator, you may test whether one set is a subset of another set (⊆ operator is set algebra).

{ 1, 2 } <= { 1, 2 }true

Every set is a subset of itself.

{ 1, 2 } == { 1, 2 }true

== tests whether two sets are identical.

{ 1, 2 } != { 1, 2 }false

!= tests whether two sets are different.

{ 1, 2, 3 } >= { 1, 2 }true

>= operator tests whether one set is a superset of another set (⊇ operator in set algebra).

{ 1, 2 } >= { 1, 2 }true

Every set is a superset of itself.

{ 1, 2, 3 } > { 1, 2 }true

> operator tests whether one set is a proper superset of another set (⊃ operator in set algebra).

{ 1, 2 } > { 1, 2 }false

A non-empty set in not a proper superset of itself.

{ 1, 2 } > {}true

Every non-empty set is a proper superset of an empty set.

{} > {}false

Empty set is not a proper subset of itself.

Union

ExpressionResultComment
{ 1, 2 } + { 2, 3 }{ 1, 2, 3 }

Sets may be unified using the + operator (∪ in set algebra).

{ 1, 2.0 } + { 2, 3 }{ 1, 2.0, 3 }

Integers, doubles, and timestamps are merged when unifying sets.

{ 1, 2 } + { 2, 3 } + { 4 }{ 1, 2, 3, 4 }

Unions may be chained.

{ 1, 2 } + {}{ 1, 2 }

Unification with empty set has no effect on the original set.

{ 1, 2 } + { "2", "3" }{ 1, 2, "2", "3" }

Strings and numbers are handled as different values.

Difference

ExpressionResultComment
{ 1, 2 } - { 2, 3 }{ 1 }

Set difference may be obtained using the - operator (operator is set algebra).

{ 1, 2 } - { 2.0, 3.0 }{ 1 }

Integers, doubles, and timestamps are considered equal if they equal in values.

{ 1, 2 } - {}{ 1, 2 }

Subtracting an empty set has no effect.

{ 1, 2 } - { 1 } - { 2 }{}

Chaining of set subtractions works from left to rights.

{ 1, 2 } - ({ 1, 2 } - { 2 }){ 2 }

Parenthesizing also works.

Intersection

ExpressionResultComment
{ 1, 2 } & { 2, 3 }{ 2 }

Set intersection may be obtained using the & operator.

{ 1, 2 } & { 2.0, 3.0 }{ 2 }

Integers, doubles, and timestamps are considered equal if they equal in values.

{ 1, 2 } & {"1", "2"}{}

Strings and numbers are handled as different values.

{"a", { 1, 2 }} & {"b", { 1, 2 }}{{1,2}}

Works with subsets.

Symmetric difference

ExpressionResultComment
{ 1, 2 } / { 2, 3 }{ 1, 3 }

Symmetric difference of sets may be obtained using the / operator.

{ 1, 2 } / { 2.0, 3.0 }{ 1, 3 }

Integers, doubles, and timestamps are considered equal if they equal in values.

{ 1, 2 } / {"1", "2"}{1, 2, "1", "2"}

Strings and numbers are handled as different values.

{"a", { 1, 2 }} / {"b", { 1, 2 }}{"a", "b"}

Works with subsets.

Arrays

Notation

ExpressionResultComment
[]

Empty array.

[1, 2, 3]

Array containing three integers.

[1, 2.0, false, "foo", null]

Arrays may contain values of different types.

[[1,2], [2,3]]

Arrays may be nested.

Value Containment

ExpressionResultComment
2 in [ 1, 2, 3 ]true

Using the in operator, you may test whether a value is contained in a given array.

4 not in [ 1, 2, 3 ]true

The in operator may be negated for better readability.

2.0 in [ 1, 2, 3 ]true

There is no difference between integers, doubles, and timestamps when testing containment.

"2" in [ 1, 2, 3 ]false

There is a difference between numbers and strings.

Comparison

ExpressionResultComment
[ 1, 2 ] == [ 1, 2 ]true

== tests whether two arrays are identical.

[ 1, 2 ] != [ 1, 2 ]false

!= tests whether two arrays are different.

[ 1, 2 ] != [ 2, 1 ]true

Order of the elements must be the same in both arrays for equality

Concatenation

ExpressionResultComment
[ 1, 2 ] + [ 3, 4 ][ 1, 2, 3, 4 ]

Arrays may be concatenated using the + operator.

[ 1 ] + [ 2, 3 ] + [ 4 ][ 1, 2, 3, 4 ]

Concatenations may be chained.

[ 1, 2 ] + [][ 1, 2 ]

Concatenating an empty array has no effect on the original array.

Indexing and Slicing

ExpressionResultComment
[ "a", "b", "c", "d" ][1]"b"

Value can be accessed by their index (starting from 0).

[ "a", "b", "c", "d" ][10]null

Requesting an index outside the array boundaries results in null.

[ "a", "b", "c", "d" ][-1]"d"

Negative indices are interpreted as counting from the end of the array. -1 therefore means the last element.

[ "a", "b", "c", "d" ][1:3]["b", "c"]

It is possible to get a sub-array between two indices

[ "a", "b", "c", "d" ][1:]["b", "c", "d"]

If the second index is omitted, all elements until the end of the array are returned.

Logical Operators

Negation (NOT)

ExpressionComment
not 'a' == 'b''a' != 'b'
not 'a' > 'b''a' <= 'b'
not truefalse
not falsetrue

Implicit conversion to boolean (for advanced uses only!):

ExpressionResultComment
not -1false

Negative numbers are truthy.

not 0true

Zero numbers are falsy.

not 1.23false

Positive numbers are truthy.

not ""true

Empty strings are falsy.

not "foo"false

Non-empty strings are truthy.

not {}true

Empty sets are falsy.

not {1,2,3}false

Non-empty sets are truthy.

not nulltrue

null is falsy.

Disjunction (OR)

ExpressionabcResultComment
'a' > 'b' or 'a' > 'c'123false

If both operands are false, false is returned.

'a' > 'b' or 'a' > 'c'213true

If at least one of boolean operands is true, the result is true.

'a' > 'b' or 'a' > 'c'231true

If at least one of boolean operands is true, the result is true.

'a' > 'b' or 'a' > 'c'312true

If- both the operands are true, the result is true.

Advanced uses: Implicit conversion to boolean.

ExpressionResultComment
"foo" or "bar""foo"

If the first operand truthy, it is returned.

"" or falsefalse

If the first operand is falsy, the second operand is returned.

false or """"

If the first operand is falsy, the second operand is returned.

Conjunction (AND)

ExpressionabcResultComment
'a' > 'b' and 'a' > 'c'123false

If both operands are false, false is returned.

'a' > 'b' and 'a' > 'c'213false

If at least one of boolean operands is false, the result is false.

'a' > 'b' and 'a' > 'c'231false

If at least one of boolean operands is false, the result is false.

'a' > 'b' and 'a' > 'c'312true

If both the operands are true, the result is true.

Advanced uses: Implicit conversion to boolean.

ExpressionResultComment
"foo" and "bar""bar"

If the first operand truthy, the second operand is returned.

"" and false""

If the first operand is falsy, it is returned.

false and ""false

If the first operand is falsy, it is returned.

Conditional Operator

ExpressionabResultComment
if 'a' > 'b' then "foo" else "bar"105"foo"

then-value is returned if the condition is satisfied.

if 'a' < 'b' then "foo" else "bar"105"bar"

else-value is returned if the condition is not satisfied.

if 'a' < 'b' then "foo"error

else clause must always be present.

if 'a' < 'b' then "foo" else (if 'a' > 'b' then "bar" else "bah")55"bah"

if-else statements may be nested using parentheses.

ExpressionResultComment
if -1 then "foo" else "bar""foo"

Negative numbers are truthy.

if 0 then "foo" else "bar""bar"

Zero numbers are falsy.

if 1.23 then "foo" else "bar""foo"

Positive numbers are truthy.

if "" then "foo" else "bar""bar"

Empty strings are falsy.

if "bah" then "foo" else "bar""foo"

Non-empty strings are truthy.

if {} then "foo" else "bar""bar"

Empty sets are falsy

if {1,2,3} then "foo" else "bar""foo"

Non-empty sets are truthy.

if null then "foo" else "bar""bar"

null is falsy.

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