I have created a table in Backendless and I would like to know if I can compose the table id with 2 fields like in a SQL table. Is it possible?
Backendless automatically assigns a unique identifier to every inserted object. There is a system column named "objectId". Additionally, you can assign constraints to your table which can be:
Indexed
Not null
Unique
.
Related
I have an old application I am supporting that uses a Microsoft Access database. The original table design did not add primary keys to every table. I am working on a migration program that among other things is adding and filling in a new primary key field (GUID) when needed.
This is happening in three steps:
Add a new guid field with no constraints
Fill the field with new unique guids
Add the primary key constraints
My problem is setting the unique guids when the table has duplicate rows. Here is my code to set the guids.
Query.SQL.Add('SELECT * FROM ' + TableName);
Query.Open;
while Query.Eof = false do
begin
Query.Edit;
Query.FieldByName(NewPrimaryKeyFieldName).AsGuid := TGuid.NewGuid;
Query.Post;
Query.Next;
end;
FireDac generates an update statement that contains a where clause with all the original fields/values in the row (since there is no unique field for it to use). However, because the rows are complete duplicates the statement still updates two rows.
FireDac correctly errors with this message
Update command updated [2] instead of [1] record.
I can open up the database in Access and delete the duplicate records or assign them a unique guid by editing the table. I would like my conversion tool to automatically do this.
Is there some way to work with these duplicate rows in FireDac? Either to update just one at a time, or to delete just one of them?
In my opinion there is no way to do it with just one SQL Statement.
I would do this:
1. Copy the whole table without duplicates by using a new temp table
SELECT DISTINCT * FROM <TABLENAME>
Add the Keys
Delete old table content and copy new content from new table
Notes:
The DB Should be unavailable for everyone else for that Operation
2. Make BACKUP before
In my iOS app,I want to change the column data type in database.
ALTER TABLE XXX ALTER COLUMN myColumn INT.
I always get 'near ALTER Syntax error'
How to resolve the problem?
Thanks for your help.
You cannot change the column type. You can create a new table, using the correct data type for the column this time, and then select data from the old table and insert it into the new table. The full procedure is outlined in the ALTER TABLE documentation:
Remember the format of all indexes and triggers associated with table X. This information will be needed in step 7 below. One way to do this is to run a query like the following: SELECT type, sql FROM sqlite_master WHERE tbl_name='X'.
Use CREATE TABLE to construct a new table "new_X" that is in the desired revised format of table X. Make sure that the name "new_X" does not collide with any existing table name, of course.
Transfer content from X into new_X using a statement like: INSERT INTO new_X SELECT ... FROM X.
If foreign key constraints are enabled, disable them using PRAGMA foreign_keys=OFF.
Drop the old table X: DROP TABLE X.
Change the name of new_X to X using: ALTER TABLE new_X RENAME TO X.
Use CREATE INDEX and CREATE TRIGGER to reconstruct indexes and triggers associated with table X. Perhaps use the old format of the triggers and indexes saved from step 1 above as a guide, making changes as appropriate for the alteration.
If foreign key constraints were originally enabled (prior to step 4) then run PRAGMA foreign_key_check to verify that the schema change did not break any foreign key constraints, and run PRAGMA foreign_keys=ON to re-enable foreign key constraints.
If any views refer to table X in a way that is affected by the schema change, then drop those views using DROP VIEW and recreate them with whatever changes are necessary to accommodate the schema change using CREATE VIEW.
Note, SQLite uses type affinity (the column definition doesn't alter what type of data you insert into the table). So if you change the data type, you'll want to change the data, too.
ALTER TABLE table ADD newColumn INTEGER;
UPDATE table SET newColumn = oldColumn;
I have these 2 tables
CREATE TABLE "QuestionWithAnswer" ("Date" DATETIME PRIMARY KEY NOT NULL , "Question" TEXT, "Answer" TEXT, "UserAnswer" TEXT, "IsCorrect" BOOL)
CREATE TABLE "Records" ("id" INTEGER PRIMARY KEY ,"DateWithTime" DATETIME,"UserGivenAnswer" TEXT DEFAULT (null) ,"Correct" TEXT DEFAULT (null) ,"Question_ID" TEXT)
i want to join them on date and retreive records and show them in tableview in ios.
The following query will get all the columns in the database where the dates match: SELECT * FROM QuestionWithAnswer AS Q JOIN Records AS R ON Q.Date=R.DateWithTime;
Next thing is up to you to create objects of the two tables and parse the fetched SQLite data into the right fields of the respective object.
After that, create a UITableVIew and set the viewcontroller (or a model) as the datasource and just use a list (NSArray, NSDictionary, ..) containing the fetched objects to populate that tableview. There are lots of good tutorials on the net on how to do this.
For your info, this kind of question is quite broad and you cannot expect a full-featured answer since that means creating a lot of code which really isn't the purpose of SO.
I have a database I would like to convert to use UUID's as the primary key in postgresql.
I have roughly 30 tables with deep multi-level associations. Is there an 'easy' way to convert all current ID's to UUID?
From this: https://coderwall.com/p/n_0awq, I can see that I could alter the table in migration. I was thinking something like this:
for client in Client.all
# Retrieve children
underwritings = client.underwritings
# Change primary key
execute 'ALTER TABLE clients ALTER COLUMN id TYPE uuid;'
execute 'ALTER TABLE clients ALTER COLUMN id SET DEFAULT uuid_generate_v1();'
# Get new id - is this already generated?
client_id = client.id
for underwriting in underwritings
locations = underwriting.locations
other_record = underwriting.other_records...
execute 'ALTER TABLE underwritings ALTER COLUMN id TYPE uuid;'
execute 'ALTER TABLE underwritings ALTER COLUMN id SET DEFAULT uuid_generate_v1();'
underwriting.client_id = client_id
underwriting.saved
underwriting_id = underwriting.id
for location in locations
buildings = location.buildings
execute 'ALTER TABLE locations ALTER COLUMN id TYPE uuid;'
execute 'ALTER TABLE locations ALTER COLUMN id SET DEFAULT uuid_generate_v1();'
location.undewriting_id = underwriting_id
location.save
location_id = location.id
for building in buildings
...
end
end
for other_record in other_records
...
end
...
...
end
end
Questions:
Will this work?
Is there an easier way to do this?
Will child records be retrieved properly as long as they are retrieved before the primary key is changed?
Will the new primary key be already generated as soon as the alter table is called?
Thanks very much for any help or tips in doing this.
I found these to be quite tedious. It is possible to use direct queries to PostgreSQL to convert table with existing data.
For primary key:
ALTER TABLE students
ALTER COLUMN id DROP DEFAULT,
ALTER COLUMN id SET DATA TYPE UUID USING (uuid(lpad(replace(text(id),'-',''), 32, '0'))),
ALTER COLUMN id SET DEFAULT uuid_generate_v4()
For other references:
ALTER TABLE students
ALTER COLUMN city_id SET DATA TYPE UUID USING (uuid(lpad(replace(text(city_id),'-',''), 32, '0')))
The above left pads the integer value with zeros and converts to a UUID. This approach does not require id mapping and if needed old id could be retrieved.
As there is no data copying, this approach works quite fast.
To handle these and more complicated case of polymorphic associations please use https://github.com/kreatio-sw/webdack-uuid_migration. This gem adds additional helpers to ActiveRecord::Migration to ease these migrations.
I think trying to do something like this through Rails would just complicate matters. I'd ignore the Rails side of things completely and just do it in SQL.
Your first step is grab a complete backup of your database. Then restore that backup into another database to:
Make sure that your backup works.
Give you a realistic playpen where you can make mistakes without consequence.
First you'd want to clean up your data by adding real foreign keys to match all your Rails associations. There's a good chance that some of your FKs will fail, if they do you'll have to clean up your broken references.
Now that you have clean data, rename all your tables to make room for the new UUID versions. For a table t, we'll refer to the renamed table as t_tmp. For each t_tmp, create another table to hold the mapping from the old integer ids to the new UUID ids, something like this:
create table t_id_map (
old_id integer not null,
new_id uuid not null default uuid_generate_v1()
)
and then populate it:
insert into t_id_map (old_id)
select id from t_tmp
And you'll probably want to index t_id_map.old_id while you're here.
This gives us the old tables with integer ids and a lookup table for each t_tmp that maps the old id to the new one.
Now create the new tables with UUIDs replacing all the old integer and serial columns that held ids; I'd add real foreign keys at this point as well; you should be paranoid about your data: broken code is temporary, broken data is usually forever.
Populating the new tables is pretty easy at this point: simply use insert into ... select ... from constructs and JOIN to the appropriate t_id_map tables to map the old ids to the new ones. Once the data has been mapped and copied, you'll want to do some sanity checking to make sure everything still makes sense. Then you can drop your t_tmp and t_id_map tables and get on with your life.
Practice that process on a copy of your database, script it up, and away you go.
You would of course want to shut down any applications that access your database while you're doing this work.
Didn't want to add foreign keys, and wanted to to use a rails migration. Anyways, here is what I did if others are looking to do this (example for 2 tables, I did 32 total):
def change
execute 'CREATE EXTENSION "uuid-ossp";'
execute <<-SQL
ALTER TABLE buildings ADD COLUMN guid uuid DEFAULT uuid_generate_v1() NOT NULL;
ALTER TABLE buildings ALTER COLUMN guid SET DEFAULT uuid_generate_v1();
ALTER TABLE buildings ADD COLUMN location_guid uuid;
ALTER TABLE clients ADD COLUMN guid uuid DEFAULT uuid_generate_v1() NOT NULL;
ALTER TABLE clients ALTER COLUMN guid SET DEFAULT uuid_generate_v1();
ALTER TABLE clients ADD COLUMN agency_guid uuid;
ALTER TABLE clients ADD COLUMN account_executive_guid uuid;
ALTER TABLE clients ADD COLUMN account_representative_guid uuid;
SQL
for record in Building.all
location = record.location
record.location_guid = location.guid
record.save
end
for record in Client.all
agency = record.agency
record.agency_guid = agency.guid
account_executive = record.account_executive
record.account_executive_guid = account_executive.guid unless account_executive.blank?
account_representative = record.account_representative
record.account_representative_guid = account_representative.guid unless account_representative.blank?
record.save
end
execute <<-SQL
ALTER TABLE buildings DROP CONSTRAINT buildings_pkey;
ALTER TABLE buildings DROP COLUMN id;
ALTER TABLE buildings RENAME COLUMN guid TO id;
ALTER TABLE buildings ADD PRIMARY KEY (id);
ALTER TABLE buildings DROP COLUMN location_id;
ALTER TABLE buildings RENAME COLUMN location_guid TO location_id;
ALTER TABLE clients DROP CONSTRAINT clients_pkey;
ALTER TABLE clients DROP COLUMN id;
ALTER TABLE clients RENAME COLUMN guid TO id;
ALTER TABLE clients ADD PRIMARY KEY (id);
ALTER TABLE clients DROP COLUMN agency_id;
ALTER TABLE clients RENAME COLUMN agency_guid TO agency_id;
ALTER TABLE clients DROP COLUMN account_executive_id;
ALTER TABLE clients RENAME COLUMN account_executive_guid TO account_executive_id;
ALTER TABLE clients DROP COLUMN account_representative_id;
ALTER TABLE clients RENAME COLUMN account_representative_guid TO account_representative_id;
SQL
end
Im using sqlite db in a sample rails application. From my users table, i have cleared all records using User.delete_all. But now whenever i insert a new record in the table using User.create, the id is starting at a value which is one more than the id of the last record which was there in the table. For example, if my table had 5 records and i cleared all, then when i do User.create, its starting at id 6.
Is there any way i can make the id start from 1 again ?
Thank You
Similar question : How to reset a single table in rails? . We can run the following at rails console to reset id column to 1 for a sqlite table
ActiveRecord::Base.connection.execute("DELETE from sqlite_sequence where name = '<table_name>'")
You seem to have autoincrement turned on for the id column.
Sqlite handles these values in an internal table called sqlite_sequence. You could reset the id for a particular autoincrement-enabled table by querying:
UPDATE "sqlite_sequence" SET "seq" = 0 WHERE "name" = $YOURTABLENAME
However, this is not a good idea because the autoincrement functionality is intended to be used in a way that the user does not influence its algorithm. Ideally, you should not care about the actual value of your id but consider it only as a unique identifier for a record.