Pagination

Walkable supports SQL pagination you’ve been familiar with: OFFSET, LIMIT and ORDER BY.

In query parameters

You can specify how to paginate your query by providing any combination of pagination keywords, namely: :offset, :limit, :order-by.

  • No pagination:

  • Query

  • SQL output

`[{(:farmers/list {})
   [:farmer/number :farmer/name]}]]
;; the same as:
`[{:farmers/list
   [:farmer/number :farmer/name]}]]
SELECT farmer.number, farmer.name FROM farmer

  • Pagination with :limit and :order-by:

  • Query

  • SQL output

`[{(:farmers/list {:limit 3 :order-by :farmer/number})
   [:farmer/number :farmer/name]}]]
SELECT farmer.number, farmer.name
 FROM farmer
 LIMIT 3
 ORDER BY farmer.number

  • Who says :order-by has to be simple?

  • Query

  • SQL output

`[{(:farmers/list {:order-by [:farmer/number :farmer/name :desc :nils-first]})
   [:farmer/number :farmer/name]}]]
SELECT farmer.number, farmer.name
FROM farmer
ORDER BY farmer.number, farmer.name DESC NULLS FIRST

Validator and default value

Pagination can be a bomb itself if you let client apps specify whatever parameters. You can just tell Walkable how pagination parameters must comply and default values when provided parameters fail that (or not even provided at all).

Validators and default values can be declared for each root or join in the registry.

Offset and limit

For offset and limit, value of default must be a number while validator is a function that check if the supplied parameter satisfies your constraint.

  • Registry

  • Query

  • SQL output

{:key :farmers/list
 :type :root
 :table "farmer"
 :default-offset  2
 :validate-offset #(<= 0 % 100)}
`[{(:farmers/list {:offset 9999})
   [:farmer/number :farmer/name]}]]

Default value 2 will be used because the supplied one (9999) fails the validator:

SELECT farmer.number, farmer.name
FROM farmer
OFFSET 2
Walkable will check if the supplied argument is an integer first, so you don’t have to do it in your validator functions.

Order-by

For order-by, default value must be a valid order-by expression while validator is a function that check if all the supplied columns satisfies your constraint. Usually you use a simple set as your validator.

  • Registry

  • Query

  • SQL output

{:key :farmers/list
 :type :root
 :table "farmer"
 :default-order-by [:farmer/number :asc]
 :validate-order-by #{:farmer/number :farmer/yob}}
`[{(:farmers/list {:order-by [:farmer/number :desc :farmer/yob :nils-first]})
   [:farmer/number :farmer/name]}]]

Supplied columns in :order-by satisfy the validator.

SELECT farmer.number, farmer.name
FROM farmer
ORDER BY farmer.number DESC, farmer.yob NULLS FIRST

  • Registry

  • Query

  • SQL output

{:key :farmers/list
 :type :root
 :table "farmer"
 :default-order-by [:farmer/number :asc]
 :validate-order-by #{:farmer/number :farmer/yob}}
`[{(:farmers/list {:order-by [:farmer/number :desc :farmer/name :nils-first]})
   [:farmer/number :farmer/name]}]]

The column :farmer/name in :order-by fails the validator. Default value will be used:

SELECT farmer.number, farmer.name
FROM farmer
ORDER BY farmer.number ASC