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Explain row realization in more detail

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Sean Corfield 2020-07-08 22:55:50 -07:00
parent e750f20df8
commit 574280dc1d
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doc/getting-started.md
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@ -230,23 +230,36 @@ Any operation that can perform key-based lookup can be used here without creatin
This means that `select-keys` can be used to create regular Clojure hash map from (a subset of) columns in the row, without realizing the row, and it will not implement `Datafiable` or `Navigable`.
If you wish to create a Clojure hash map that supports that lazy navigation, you can call `next.jdbc.result-set/datafiable-row`, passing in the current row, a `connectable`, and an options hash map, just as you passed into `plan`. Compare the difference in output between these three expressions:
If you wish to create a Clojure hash map that supports that lazy navigation, you can call `next.jdbc.result-set/datafiable-row`, passing in the current row, a `connectable`, and an options hash map, just as you passed into `plan`. Compare the difference in output between these four expressions:
```clojure
;; selects specific keys (as simple keywords):
user=> (into []
(map #(select-keys % [:id :product :unit_price :unit_cost :customer_id]))
(jdbc/plan ds ["select * from invoice where customer_id = ?" 100]))
;; selects specific keys (as qualified keywords):
user=> (into []
(map #(select-keys % [:invoice/id :invoice/product
:invoice/unit_price :invoice/unit_cost
:invoice/customer_id]))
(jdbc/plan ds ["select * from invoice where customer_id = ?" 100]))
;; selects specific keys (as qualified keywords -- ignoring the table name):
user=> (into []
(map #(select-keys % [:foo/id :bar/product
:quux/unit_price :wibble/unit_cost
:blah/customer_id]))
(jdbc/plan ds ["select * from invoice where customer_id = ?" 100]))
;; do not do this:
user=> (into []
(map #(into {} %))
(jdbc/plan ds ["select * from invoice where customer_id = ?" 100]))
;; do this if you just want realized rows with default qualified names:
user=> (into []
(map #(rs/datafiable-row % ds {}))
(jdbc/plan ds ["select * from invoice where customer_id = ?" 100]))
```
The latter produces a vector of hash maps, just like the result of `execute!`, where each "row" follows the case conventions of the database, the keys are qualified by the table name, and the hash map is datafiable and navigable.
The latter produces a vector of hash maps, just like the result of `execute!`, where each "row" follows the case conventions of the database, the keys are qualified by the table name, and the hash map is datafiable and navigable. The third expression produces a result that looks identical but has stripped all the metadata away: it has still called `rs/datafiable-row` to fully-realize a datafiable and navigable hash map but it has then "poured" that into a new, empty hash map, losing the metadata.
In addition to the hash map operations described above, the abstraction over the `ResultSet` can also respond to a couple of functions in `next.jdbc.result-set`: