I have a NSTableView with bindings.
Each row of the table is a Core Data Entity named Entry.
An Entry can have several CustomValue. Each CustomValue should be displayed in its own column in the Table.
I had 2 choices to model this in Core Data:
Choice 1: Either model a dictionary as a Transformable, where the key is the column name & the value is the CustomValue.
Choice 2: Either use a To-Many relation ship Entry <-->> CustomValue <--> key & value where key is a Column entity.
I chose the second solution based on this: Best practice? - Array/Dictionary as a Core Data Entity Attribute
BUT my problem is : with Choice 2, how can I bind the data to the column?
With Choice 1, I could do something like this:
textField.bind(NSBindingName.value, to: cellView, withKeyPath: "objectValue.myDictionary.\(tableColumn!.identifier.rawValue).value", options: nil)
I'm currently stuck, I need to map a binding to something that would allow me to search for the proper value based on the key.
Any idea? Thanks in advance :-)
Related
I am new to Cassandra, and found below in the wikipedia.
A column family (called "table" since CQL 3) resembles a table in an RDBMS (Relational Database Management System). Column families contain rows and columns. Each row is uniquely identified by a row key. Each row has multiple columns, each of which has a name, value, and a timestamp. Unlike a table in an RDBMS, different rows in the same column family do not have to share the same set of columns, and a column may be added to one or multiple rows at any time.[29]
It said that 'different rows in the same column family do not have to share the same set of columns', but how to implement it? I have almost read all the documents in the offical site.
I can create table and insert data like below.
CREATE TABLE Emp_record(E_id int PRIMARY KEY,E_score int,E_name text,E_city text);
INSERT INTO Emp_record(E_id, E_score, E_name, E_city) values (101, 85, 'ashish', 'Noida');
INSERT INTO Emp_record(E_id, E_score, E_name, E_city) values (102, 90, 'ankur', 'meerut');
It's very like I did in the relational database. So how to create multiply rows with different columns?
I also found the offical document mentioned 'Flexible schema', how to understand it here?
Thanks very much in advance.
Column family is from the original design of Cassandra, when the data model looked like the Google BigTable or Apache HBase, and Thrift protocol was used for communication. But this required that schema was defined inside the application, and that makes access to data from many applications more problematic, as you need to update the schema inside all of them...
The CREATE TABLE and INSERT is a part of the Cassandra Query Language (CQL) that was introduced long time ago, and replaced Thrift-based implementation (Cassandra 4.0 completely removed the Thrift support). In CQL you need to have schema defined for a table, where you need to provide column name & type. If you really need to have dynamic columns, there are several approaches to that (I'll link answers that I already wrote over the time, so there won't duplicates):
If you have values of the same type, you can use one column as a name of the attribute/column, and another to store the value, like described here
if you have values of different types, you can also use one column as a name of attribute/column, and define multiple columns for values - one for each of the data types: int, text, ..., and you insert value into the corresponding columns only (described here)
you can use maps (described here) - it's similar to first or second, but mostly designed for very small number of "dynamic columns", plus have other limitations, like, you need to read the full map to fetch one value, etc.)
I have recently started learning No-SQL and Cassandra through this article. The author explains the data model through this diagram:
The author also gives the below column family example:
Book {
key: 9352130677{ name: “Hadoop The Definitive Guide”, author:” Tom White”, publisher:”Oreilly”, priceInr;650, category: “hadoop”, edition:4},
key: 8177228137{ name”” Hadoop in Action”, author: “Chuck Lam”, publisher:”manning”, priceInr;590, category: “hadoop”},
key: 8177228137{ name:” Cassandra: The Definitive Guide”, author: “Eben Hewitt”, publisher:” Oreilly”, priceInr:600, category: “cassandra”},
}
But in that tutorial and every other tutorial I have gone through, then end up creating regular tables in cassandra. I am unable to connect the Cassandar model with what I am creating.
For example, I created a column family called Employee as below:
create columnfamily Employee(empid int primary key,empName text,age int);
Now I inserted some data and my column family looks as this:
For me this looks like a regular relational table and not like the data model the author has explained. How do I create a Employee column family where each row represents an employee with different attributes? Something like:
Employee{
101:{name:Emp1,age:20}
102:{name:Emp2,salary:1000}
102:{manager_name:Emp3,age:45}
}
}
You need to understand that in the representation using cql, is may look like regular relational table, but the internal structure of the rows in Cassandra is completely different. It is saving different set of attributes for each employee, and the nulls you can see while querying with cql is just a representation of empty/nonexistent cells.
What you trying to achieve, is unstructured data model. Cassandra started with this model, and all was working as described in the tutorial you've read, but there is an opinion that unstructured data design is unhealthy to development and makes more problems than it solves. So, after sometime, Cassandra moved to the "structured" data structure (and from thrift to cql). It doesn't mean that you have to store all attributes for all keys/rows, it doesn't mean that all the rows are have same number of attributes, it just means that you have to declare attributes before you use them.
You can achieve some kind of unstructured data modeling using Map, List, Set, etc. data types, UDT (User defined types) or just saving your data as json string and parsing it on the application side.
What you have understood is correct. Just believe it. Internally cassandra stores columns exactly like the image in your question.
Now, what you are expecting is to insert a column which is not defined while creating the Employee table. For dynamic columns, you can always use Map data types .
For example
create table Employee(
empid int primary key,
empName text,
age int,
attributes Map<text,text>);
To add new attributes you can use below queries.
UPDATE Employee SET attributes = { manager_name : Emp3, age:45 } WHERE empid = 102;
Update -
another way to to create a dynamic column model is as below
create table Employee(
empid int primary key,
empName text,
attribute text,
attributevalue text,
primary key (empid,empName,attribute)
);
Lets take few inserts -
insert into Employee (empid,empName,attribute,attributevalue) values (102,'Emp1','age','25') ;
insert into Employee (empid,empName,attribute,attributevalue) values (102,'Emp1','manager','emp2') ;
insert into Employee (empid,empName,attribute,attributevalue) values (102,'Emp1','department','hr') ;
this data structure will create a wide row, and behaves like dynamic column. you can see primary key empid and name is common for all three rows, only attribute and value will change.
Hope this will help
Cassandra uses a special primary key called compositie key. This is the representation of the partitions. This is also one reason why cassandra scales well. The composite key is used to determine the nodes on which the rows are stored.
The result in your console may be a result set of rows, but the intern organization of cassandra is differnt from that. Have you ever tried to query a table without an primary key? You will quickly see that you can't query that flexible (because of the partitioning).
After that you will understand why we have to use a query-first design aproach for cassandra. This is completely different from RDBBS.
I want to retrieve all items within a specific column of a table.
In this scenario, I have 2 tables, The first table contains a primary key, and the second table contains a foreign key. a 1 to many relationship is set up for the tables respectively.
I want a function/way of retrieving all items within a column in table 2 that has a foreign key that matches the primary key in table 1.
One way of doing this is through a VLOOKUP, though surely through using DAX, or some other function set, I can exploit the relationship I have made in the DataModel to make this easier for me to do.
Why don't you just get the required data from the DB with a proper SELECT statement? Something like
SELECT column
FROM t1, t2
WHERE t1.key = t2.fkey
AND t1.key = 'whatever you search for';
Then you should get the data you want.
I'm newbie design cassandra data model and I need some help to think out the box.
Basically I need a hierarchical table, something pretty standard when talking about Employee.
You have a employee, say Big Boss, that have a list of employee under him.
Something like:
create table employee(id timeuuid, name text, employees list<employee>, primary key(id));
So, is there a way to model a hierarchical model in Cassandra adding the table type itself, or even another approach?
When trying this line above it give me
Bad Request: line 1:61 no viable alternative at input 'employee'
EDITED
I was thinking about 2 possibilities:
Add an uuid instead and in my java application find each uuid Employee when bringing up the "boss".
Working with Map, where the uuid is the id itself and my text would be the entire Row, then in my java application get the maps, convert each "text" employee into a Employee entity and finally return the whole object;
It really depends on your queries...one particular model would only be good for a set of queries, but not others.
You can store ids, and look them up again at the client side. This means n extra queries for each "query". This may or may not be a problem, as queries that hit a partition are fast. Using a map from id to name is also an option. This means you do extra work and denormalise the names into the map values. That's also valid. A third option is to use a UDT (user defined type). You could then have a list or set or even map. In cassandra 2.1, you could index the map keys/ values as well, allowing for some quite flexible querying.
https://www.datastax.com/documentation/cql/3.1/cql/cql_using/cqlUseUDT.html
One more approach could be to store a person's details as id, static columns for their attributes, and have "children" as columns in wide row format.
This could look like
create table person(
id int primary key,
name text static,
age int static,
employees map<int, employeeudt>
);
http://www.datastax.com/documentation/cql/3.1/cql/cql_reference/refStaticCol.html
Querying this will give you rows with the static properties repeated, but on disk, it's still held once. You can resolve the rest client side.
I'm new to core data.
I have a data model, in which there are two tables and a 1-N relationship between them.
The application downloads all data from a service and saves the data in each table.
In addition, the tables are related and therefore want to do this:
a) Obtain all elements of the table2, which satisfies certain conditions.
b) For each element in table2, the identifier look table1 / save the table1´s id.
c) I get the item in Table 1 which meets the requirements ID.
d) I relate to Table 2 with 1.
I'm not capable for doing this. :(
I do not know if this method for make a relation between tables in this way is good or no.
This is sort of difficult to answer. If you think about Core Data as an SQL table you'll just get yourself into difficulty.
Core data isn't about joining and searching tables, it's about an object graph. An object has relationships to another object which has an inverse relationship to the other object. Essentially, what you should be looking to do is:
This is a fetch request of the entity which you are storing in table 2 subject to a predicate which defines your conditions.
You don't actually deal with ids directly in Core Data. You hardly ever deal with the keys directly.
Step 1 returned a collection of objects, and you can run a further predicate on this to filter it.
That is what the inverse relationship is for.
I know this doesn't answer your actual question. I'm trying to get you to think of your Core Data store as a collection of objects related to each other rather than as a bunch of linked tables.