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pagination.md

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

Inside a query's parameters

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

  • No pagination: {% mdtabs title="Query" %}
`[{(:farmers/all {})
   [:farmer/number :farmer/name]}]]
;; the same as:
`[{:farmers/all
   [:farmer/number :farmer/name]}]]

{% mdtab title="SQL output" %}

SELECT farmer.number, farmer.name FROM farmer

{% endmdtabs %}

  • Pagination with :limit and :order-by: {% mdtabs title="Query" %}
`[{(:farmers/all {:limit 3 :order-by :farmer/number})
   [:farmer/number :farmer/name]}]]

{% mdtab title="SQL output" %}

SELECT farmer.number, farmer.name
 FROM farmer
 LIMIT 3
 ORDER BY farmer.number

{% endmdtabs %}

  • Who says :order-by has to be simple? {% mdtabs title="Query" %}
`[{(:farmers/all {:order-by [:farmer/number :farmer/name :desc :nils-first]})
   [:farmer/number :farmer/name]}]]

{% mdtab title="SQL output" %}

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

{% endmdtabs %}

Validator and default value

Pagination can be a bomb 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 respective default values can be declared for each idents it in :floor-plan.

Offset and limit

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

{% mdtabs title="Floor-plan" %}

{:idents
 {:farmers/all "farmer"}

 :pagination-fallbacks
 {:farmers/all
  {:offset {:default  2
            :validate #(<= 0 % 100)}}}}

{% mdtab title="Query" %}

`[{(:farmers/all {:offset 9999})
   [:farmer/number :farmer/name]}]]

{% mdtab title="SQL output" %}

Will use default value 2 because 9999 fails the validator:

SELECT farmer.number, farmer.name
FROM farmer
OFFSET 2

{% endmdtabs %}

{% hint style="info" %} Walkable will check if the supplied argument is an integer first, so you don't have to do it in your validator functions. {% endhint %}

Order-by

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

{% mdtabs title="Floor-plan" %}

{:idents
 {:farmers/all "farmer"}

 :pagination-fallbacks
 {:farmers/all
  {:order-by {:default  [:farmer/number :asc]
              :validate #{:farmer/number :farmer/yob}}}}}

{% mdtab title="Query" %}

`[{(:farmers/all {:order-by [:farmer/number :desc :farmer/yob :nils-first]})
   [:farmer/number :farmer/name]}]]

{% mdtab title="SQL output" %}

Supplied columns in :order-by satisfy the validator.

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

{% endmdtabs %}

{% mdtabs title="Floor-plan" %}

{:idents
 {:farmers/all "farmer"}

 :pagination-fallbacks
 {:farmers/all
  {:order-by {:default  [:farmer/number :asc]
              :validate #{:farmer/number :farmer/yob}}}}}

{% mdtab title="Query" %}

`[{(:farmers/all {:order-by [:farmer/number :desc :farmer/name :nils-first]})
   [:farmer/number :farmer/name]}]]

{% mdtab title="SQL output" %}

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

{% endmdtabs %}