relation

问题 Would it be possible to do relation in this sense in firestore ? whereby I want to relate a field in collection to another field in another collection Eg: I have 2 different collection - tracking and venue tracking <-- collection 1. document(xyz123) device_unique_identifier = "abcd1234" timestamp = 10/09/2019 10:00 2. document(xyz567) device_unique_identifier = "efgh3456" timestamp = 10/09/2019 11:00 venue <-- collection 1. document(zyx123) name = "room A" device_unique_identifier = "abcd1234

问题 I wanted to overwrite some default value in a relation with some specific values and stumbled upon some behavior I don't quite understand. (clojure.set/join #{ {:a 1 :b nil :c "2"} {:a 2 :b "2" :c nil} {:a 3 :b 1 :c 5} } #{ {:a 1 :b 44} {:a 3 :b 11 :c 55} } {:a :a}) resolves to #{{:a 3, :c 5, :b 1} {:c "2", :a 1, :b nil}} . If I flip the arguments I get the same result. (clojure.set/join #{ {:a 1 :b 44} {:a 3 :b 11 :c 55} } #{ {:a 1 :b nil :c "2"} {:a 2 :b "2" :c nil} {:a 3 :b 1 :c 5} } {:a

问题 In my case, nodes are connected with one or more relations of type similar . This relation has two attributes fldName and value . For example: (x)-[r:similar {fldName:'e-mail', value: 'name@x.y.com' }]->(y) (x)-[r:similar {fldName:'phone', value: '123-45-67' }]->(y) (q)-[r:similar {fldName:'e-mail', value: 'other@x.y.com' }]->(p) (q)-[r:similar {fldName:'phone', value: '891-23-45' }]->(p) (s)-[r:similar {fldName:'e-mail', value: 'neo@x.y.com' }]->(t) Questions: 1) What is the query to return

问题 How do I use a named association in the where clause associated with a join? class Pet < ActiveRecord::Base belongs_to :owner end class Owner < ActiveRecord::Base has_many :dogs, :class_name => 'Pet', :foreign_key => :owner_id end Owner.joins(:dogs).where(:dogs => {:name => 'fido'}).to_sql generates: "SELECT `owners`.* FROM `owners` INNER JOIN `pets` ON `pets`.`owner_id` = `owners`.`id` WHERE (`dogs`.`name` = 'fido')" Note that the WHERE clause is looking in the dogs table instead of the pets

问题 Suppose I have the following relations: Branch (branchNo(PK), street, city, postcode) Staff (staffNo(PK), fName, lName, sex, branchNo(FK)) Not that it matters for this question, but PK = primary key & FK = foreign key How would I write the relational algebra for the following query: List the names of all female staff that work in Glasgow. My attempt: σ Staff.sex=F & Branch.city = Glasgow (π fName, lName, sex, branchNo (Staff) x π city, branchNo (Branch)) I know that my selection (σ) statement

问题 Suppose I have an model: class Post end posts = Post.where(***) puts posts.class # => ActiveRecord::Relation Then how can I get the model class name through the variable 'posts', maybe some method called model_class_name: puts posts.model_class_name # => Post Thanks :) 回答1: The #klass attribute of ActiveRecord::Relation returns the model class upon which the relation was built: arel = User.where(name: "fred") arel.klass # User To get the class's name: arel.klass.name Tested in ActiveRecord 4

问题 I got the problem to display the data from two table into JSON format and working on yii2 restful api. this is my structure database: TABLE `volunteer`( `volunteer_id` int(11) NOT NULL auto_increment, `state_id` int(11) null TABLE `state`( `state_id` int(11) NOT NULL auto_increment, `state` varchar(225) null volunteerController.php public $modelClass = 'app\models\Volunteer'; public function behaviors() { return ArrayHelper::merge(parent::behaviors(),[ 'verbs' => [ 'class' => VerbFilter:

问题 I'm really confused on simplifying this recurrence relation: c(n) = c(n/2) + n^2. So I first got: c(n/2) = c(n/4) + n^2 so c(n) = c(n/4) + n^2 + n^2 c(n) = c(n/4) + 2n^2 c(n/4) = c(n/8) + n^2 so c(n) = c(n/8) + 3n^2 I do sort of notice a pattern though: 2 raised to the power of whatever coefficient is in front of "n^2" gives the denominator of what n is over. I'm not sure if that would help. I just don't understand how I would simplify this recurrence relation and then find the theta notation

问题 How i can override manager for relations? For example: user.entry_set # django.db.models.fields.related.RelatedManager But i need for my own manager. I try this code, but this does not work: class EntryManager(models.Manager): use_for_related_fields = True Thanks. 回答1: You task requires rather hacky code which will be very hard to maintain and understand (if it's even possible). What about this? class EntryManager(models.Manager): def by_user(self, user): # do anything you want # usage Entry

问题 I can't get the onDelete to work in Doctrine2 (with YAML Mapping). I tried this relation in my Product class: oneToOne: category: targetEntity: Category onDelete: CASCADE But that doesn't work.. EDIT: I've set the ON DELETE: CASCADE manually in the database imported the YAML mapping with doctrine:mapping:import , emptied the database updated it from the schema with doctrine:schema:update and got no ON DELETE in the foreign key.. so looks like even Doctrine doesn't know how to do it lol.. 回答1: