I would like to ask how I can create a combination of unique keys in ruby Mongoid where each key is not unique but they are unique when they are together in one record (key1+key2).unique. I need to insert a data in to database but in that way when in database can exist only one record with (key1+Key2) combination
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I have the following data model for which I want to use redis as cache.
Employee: With a unique Employee_Id.
Department: With a unique Department_Id.
An employee can be part of only one department, a department can have many employees. Now, the operations the system should support are something like this.
Given a Employee_Id, find the department it is a part of.
Given a Department_Id, find the list of all it's employees.
Merge to departments, in this case employees of any one department will move to other, depending on the least no.of db operations.
I'm using DynamoDB as the persistent storage with two tables representing Employee and Department. I'm performing merge operations using dynamodb transactions, to ensure ACID.
Now, I'm planning to use redis as a cache between service and db. For each employee_Id as key, I'll store the department it is part of. For each department_id as key, I'll store the the list of members in the department. Now, for merge usecase I'll have to update the values for a no.of employee -> department mapping. For this I want to use redis transactions or operations like MSET, MGET etc.
For transactions in redis, we need to ensure that all keys are in the same hash slots. However, in our case EmployeeId(Key) are randomly generated UUID, they will not have any common regx. pattern to use for hash-tags. But, the values that they point to, i.e. Depatment_id will be common for them.
Is there any way in Redis to put keys(employee_Id) without any common regx pattern in same hash slots?
I'll put all such entries(for which I might want to perform transactions in future) in redis at the same time, hence I was thinking of appending a random string as hash-tag (between '{' and '}') to the keys but while getting value for the key, I'll not know the random the random string added, I need fetch values based on the original keys only.
I wanted to develop a simple Android app that requires a small database. I've developed a prototype with App Inventor and TinyDB, but it seems that TinyDB allows to add several records to the database with the same "tag" (this is how keys are named in TinyDB).
I am adding an extra field that autoincrements itself in every database record and using this counter as a primary key, but that's not exactly what I want. Is there a way to implement a primary key constraint for a "tag" in TinyDB?
TinyDB has no built-in way to store primary keys, but you can store an ordered list of the items where the index is the primary key. Then you just find where it is in the list to find the primary key.
If you use that system, though, you will decrease the keyspace (there will be one less possible tag out of an infinite number of possible tags that can be stored by the user.) If the user gets to create their own tags, you can prefix all of the tags they create with a symbol. No matter what tags the user enters, they will not be able to accidentally or purposely overwrite your primary key list.
How to create unique items in RethinkDB?
In MongoDb I used ensureIndex for this, eg:
userCollection.ensureIndex({email:1},{unique:true},function(err, indexName){
RethinkDB does not currently support uniqueness constraints on fields other than the primary key.
You could use an auxiliary table where the unique field is stored as the primary key in order to check for uniqueness in your application explicitly.
A relatively easy alternative would be to use a secondary index. Thus not messing with auxiliary tables.
You can than do the check in your application with the get_all method (http://rethinkdb.com/api/#js:selecting_data-get_all).
I am analyzing an Oracle database design and I am perplexed at seeing both unique keys and primary keys on the same fields. These unique-primary key pairs are consistently created on all tables. I see no reason to do this.
If I have a primary key anyway, is there a good reason to create an additional unique key on the same field?
For a table resolving a many-to-many it would be common to have a two part key (as indicated by Quassnoi). It is also quite likely to need indexes supporting access through either parent.
If you have, for example, PERSON, ADDRESS and PERSON_ADDRESS tables, your PERSON_ADDRESS table may have the primary key of (PERSON_ID, ADDRESS_ID) and a supporting index. You would also have another index on (ADDRESS_ID,PERSON_ID), and you would likely make this a UNIQUE index (as it is a unique combination of fields).
It is also possible that your DBA has some particular way of generating tables that starts with a UNIQUE index on the primary key fields followed by the creation of the PRIMARY KEY constraint. That may show up in some GUI tools in the way you suggest.
No, there is no reason to have it also as unique; when you set a column as PK you are sure that:
No NULL will be accepted for that column on INSERT or UPDATE;
Values in the whole table for that column are always UNIQUE;
so just PK is enough. Since there is a UNIQUE index for the PK column, by definition, there is no need to add any other index on that column only because queries will use the PK index whenever only that column is affected.
I believe it's impossible (PK and unique constraint on the same column[s])...
You cannot designate the same column or combination of columns as both a primary key and a unique key.
(from here, section "Restrictions on Primary Key Constraints"). Isn't it?
Oracle won't let you create multiple UNIQUE and PRIMARY KEY constraints on the same field set in the same order and will fail with ORA-02261.
If you have composite keys, you can create PRIMARY KEY on the column set in one order (PRIMARY KEY (a, b)) and a unique constraint on another (UNIQUE (b, a)).
This will parse and execute, however a single index will be used to police both constraints so it makes no sense.
Could you please post the table scripts?
Just a little theoretical background here... When modeling your table, you identify a set of keys. These keys are logically equivalent, but for practical purposes you pick one of them and call it "primary" while the rest of them become "alternate".
(In DDL SQL, a primary key is called "PRIMARY KEY", while "alternate key" is called "UNIQUE constraint".)
So, in light of that, your question is equivalent to: "is there a good reason to have two identical keys", and the answer is: "no".
That being said, you may have overlapping keys (i.e. keys that share some fields but not all), but this is usually a sign of a bad design... and the answer is: "probably not".
OTOH, if by "unique key", you actually mean "unique index", then yes, you need both of them.
Index is not a logical constraint - it is there just to allow a logical constraint such as PRIMARY KEY to perform well (and for querying, but that's a different topic).
I have to add some security for a C#/.NET WinForms/Desktop application. I am using Oracle DB back-end.
The tables are simple: User (ID,Name), Role(ID,Role), UserRole(UserID,RoleID).
I am using the windows account name to populate User table. Role table will for now just be simply 'Admin','SuperUser','BasicUser'...
Since no two people could ever possible have the same windows account name... even when I do not control these name management (netops does, hence why I want to use windows accounts so I don't have to manage it ;)). For Role table, I should again never have dupe value - I control the input, there will only be 3 (tactical app going away within year). UserRole is a join table to represent the Many-To-Many relationships of users and roles, so no surragate key is justified.
Simple question - Why bother with 'ID' (int) in the User and Role table? Any point or advantage here? Is this one of those 'I've always done it this way' type things? Or have I just not done this in awhile and forget the reason?
Names change - primary key values must not. Abigail Smith becomes Abigail Jones and the username changes but a surrogate key protects against having to cascade those changes everywhere.
If you are using a surrogate key but there is a column or combination of columns which should be unique, then enforce that using a unique index. There's a good chance you'll want indexes on your user.name and role.role columns anyway, and a unique index is more space efficient and supplies useful metadata to the optimizer. If you have a surrogate key but don't have another combination of columns that uniquely identify a row then think again whether you have your entity definition right.
One caution. Especially for very narrow tables with few access paths, you may use an index-organized table. Oracle will only allow an index organized table on the primary key, but does allow foreign keys against a unique set of columns (if it is enforced by a unique constraint, not simply a unique index).
It is possible that you'll end up with a table where a unique ID is enforced through a unique index and treated as PK by an ORM and used as the parent for foreign key relationships, but the primary key (as defined in the DB) is the rolename/username/whatever because you want that as the driver for an index-organised table.
A surrogate key is not required on intersection tables, but here are a few reasons to do so:
Consistency: If every table has a single artificial key, you always know the key name when you know the table name.
Ease Of Use: Less typing — one key means ON and WHERE clauses are shorter and thus less error-prone.
Interoperability: Some ORMs only work well with tables with a single primary key column.