假设我有一个包含属性:x/value的架构，其中:x/value是一个组件，是一个ref，并且基数很多。该架构还具有x :x/id的ID。
现在让我们说我交易以下内容：
(d/transact conn [{:x/id "1234" :x/value [{:text "test"}]}])
然后我想更新这个值，这意味着我想要替换:x/value，所以最后我有一个像这样的实体：
{:db/id <some eid>
:x/id "1234"
:x/value [{:text "replacement"}]}
我该怎么做？
到目前为止，我已尝试过以下方法：
(d/transact conn [{:x/id "1234" :x/value [{:text "replacement"}]}])
但这只是添加了一个新的参考，所以我得到了一个实体：
{:db/id <some eid>
:x/id "1234"
:x/value [{:text "test"} {:text "replacement"}]}
我认为，实现我想要的方法的一种方法是通过实体ID手动收回两个：text属性，然后为x实体执行新的添加事务。
但我想知道是否有更好的方法来做到这一点。有什么想法吗？
答案 0 :(得分：4)
您需要撤消旧值，然后使用新值更新它：
[:db/retract entity-id attribute old-value]
[:db/add entity-id attribute new-value]
请参阅http://docs.datomic.com/transactions.html
您可以在the James Bond example from Tupelo Datomic中查看更多详情。以下是创建属性的方法：
(td/transact *conn* ; required required zero-or-more
; <attr name> <attr value type> <optional specs ...>
(td/new-attribute :person/name :db.type/string :db.unique/value) ; each name is unique
(td/new-attribute :person/secret-id :db.type/long :db.unique/value) ; each secret-id is unique
(td/new-attribute :weapon/type :db.type/ref :db.cardinality/many) ; one may have many weapons
(td/new-attribute :location :db.type/string) ; all default values
(td/new-attribute :favorite-weapon :db.type/keyword )) ; all default values
假设詹姆斯把刀扔向别墅。我们需要将其从数据库中删除。
(td/transact *conn*
(td/retract-value james-eid :weapon/type :weapon/knife))
(is (= (td/entity-map (live-db) james-eid) ; lookup by EID
{:person/name "James Bond" :location "London" :weapon/type #{:weapon/wit :weapon/gun} :person/secret-id 7 } ))
一旦詹姆斯击败了诺博士，我们就需要从数据库中删除他（以及他拥有的一切）。
; We see that Dr No is in the DB...
(let [tuple-set (td/find :let [$ (live-db)]
:find [?name ?loc] ; <- shape of output tuples
:where {:person/name ?name :location ?loc} ) ]
(is (= tuple-set #{ ["James Bond" "London"]
["M" "London"]
["Dr No" "Caribbean"]
["Honey Rider" "Caribbean"] } )))
; we do the retraction...
(td/transact *conn*
(td/retract-entity [:person/name "Dr No"] ))
; ...and now he's gone!
(let [tuple-set (td/find :let [$ (live-db)]
:find [?name ?loc]
:where {:person/name ?name :location ?loc} ) ]
(is (= tuple-set #{ ["James Bond" "London"]
["M" "London"]
["Honey Rider" "Caribbean"] } )))
使用原生数据库几乎完全相同，只是不如图珀洛那么甜：
; Dr No is no match for James. He gives up trying to use guile...
; Remove it using native Datomic.
(spy :before (td/entity-map (live-db) [:person/name "Dr No"]))
(d/transact *conn*
[[:db/retract [:person/name "Dr No"] :weapon/type :weapon/guile]])
(is (= (spy :after (td/entity-map (live-db) [:person/name "Dr No"])) ; LookupRef
{:person/name "Dr No"
:location "Caribbean"
:weapon/type #{:weapon/knife :weapon/gun}}))
:before => {:person/name "Dr No",
:weapon/type #{:weapon/guile :weapon/knife :weapon/gun},
:location "Caribbean"}
:after => {:person/name "Dr No",
:weapon/type #{:weapon/knife :weapon/gun},
:location "Caribbean"}
旁注：我注意到您的示例显示了:arb/value [{:db/id 17592186045435, :content/text "tester"}]，这是一个长度为1的地图列表。这与我的示例不同：武器/类型只是一个平原一套N个项目。此输出差异是因为您使用的是Datomic的pull API。但是，这不会影响您的原始问题。
我们都知道多年来詹姆斯有很多邦德女孩。这是一个如何添加一些蜂蜜的例子，他们将其中一个降级：
(defn get-bond-girl-names []
(let [result-pull (d/pull (live-db) [:bond-girl] [:person/name "James Bond"])
bond-girl-names (forv [girl-entity (grab :bond-girl result-pull) ]
(grab :person/name (td/entity-map (live-db) (grab :db/id girl-entity))))
]
bond-girl-names))
(td/transact *conn*
(td/new-attribute :bond-girl :db.type/ref :db.cardinality/many)) ; there are many Bond girls
(let [tx-result @(td/transact *conn*
(td/new-entity {:person/name "Sylvia Trench"})
(td/new-entity {:person/name "Tatiana Romanova"})
(td/new-entity {:person/name "Pussy Galore"})
(td/new-entity {:person/name "Bibi Dahl"})
(td/new-entity {:person/name "Octopussy"})
(td/new-entity {:person/name "Paris Carver"})
(td/new-entity {:person/name "Christmas Jones"}))
tx-datoms (td/tx-datoms (live-db) tx-result)
girl-datoms (vec (remove #(= :db/txInstant (grab :a %)) tx-datoms))
girl-eids (mapv :e girl-datoms)
txr-2 (td/transact *conn*
(td/update [:person/name "James Bond"] ; update using a LookupRef
{:bond-girl girl-eids})
(td/update [:person/name "James Bond"] ; don't forget to add Honey Rider!
{:bond-girl #{[:person/name "Honey Rider"]}}))
]
(is (= (get-bond-girl-names)
["Sylvia Trench" "Tatiana Romanova" "Pussy Galore" "Bibi Dahl"
"Octopussy" "Paris Carver" "Christmas Jones" "Honey Rider"]))
; Suppose Bibi Dahl is just not refined enough for James. Give her a demotion.
(td/transact *conn*
(td/retract-value [:person/name "James Bond"] :bond-girl [:person/name "Bibi Dahl"]))
(newline)
(is (= (get-bond-girl-names) ; Note that Bibi Dahl is no longer listed
["Sylvia Trench" "Tatiana Romanova" "Pussy Galore"
"Octopussy" "Paris Carver" "Christmas Jones" "Honey Rider"] ))
)
请注意，您只能使用LookupRef [:person/name "Honey Rider"]，因为:person/name属性有:db.unique/value。如果您的:content/text不是:db.unique/value，则必须使用EID将其与父实体分离。