Overview

PhixFlow ERDs define the logical structure of the data in your application. 

The following example shows an ERD for a school. A school has entities, for example: departments, teachers, students, classrooms and so on. All these entities are represented as stream. The lines connecting the streams show the relationships between them.

The stream has a list of attributes, which represent the different data you want to record. For example, the Student stream in the ERD above has the attributes: UID, Address, IntakeYear and Name.

Attributes are configured with a data type and associated properties, for example the Name and Address attributes are labelled as "String". To make it easy to create an attribute, PhixFlow sets the type to String by default. To change the attribute to reflect the nature of your data, click on the attribute name to open it's properties, where you can set the type. For example, IntakeYear is set to Date. It is important to set the correct data type and properties before loading records into a stream, as you cannot change the data type once the stream contains records.

Identifying an Attribute

This help page refers to an attribute using the form stream.attribute, for example: Teacher.Name, Student.Name or SchoolDept.Name. 

Primary Keys and UIDs

PhixFlow expects every stream to have a special attribute, called it's primary key. The primary key contains a unique value for every record. For example, several teachers can have the same first or family name, or even both, so these attributes cannot be a primary key. When you create a stream, PhixFlow automatically adds an attribute called UID (short for Unique IDentifier) and configures it:

• as the stream's  primary key.
  If you look at the School ERD example above, you can see that every stream has an attribute called UID, and that it has the primary key icon next to it.
• as an integer; see the SchoolDept primary key properties on the right.
• to automatically create a unique number for every record in a stream.

This means PhixFlow can ensure all the UIDs have unique values. 

PhixFlow cannot ensure unique values for a primary key where:

• the name is UID but it's data type is not integer
• the name is not UID.

If your data already has a unique attribute, you can use that as the primary key. For example, the Teacher table could have a National Insurance attribute, which would be unique to a teacher. In this case, you must set up your own processes to make sure that the records in a table all have unique values in the primary key.

Display Name 

Where your stream's primary key is a UID, you usually do not want to show it to the user on a screen. For example the SchoolDept.UID will be a number, for example 1490 or 1550. Application users want to see the department name, such as Arts or Science. For this reason, when you create a stream, PhixFlow automatically adds an attribute called Name and ticks the Display Name property, as shown for the SchoolDept.Name attribute on the right. 

Attribute properties for:

    SchoolDept                         SchoolDept 
    primary key                           Name 

Relationships and Foreign Keys

Streams in PhixFlow are relational. This means you can access data in many different ways without: 

• having to duplicate data
• reorganizing the database streams themselves.

In the ERD you can connect streams by drawing a line, called a relationship. This tells PhixFlow that the two streams are related. For example, both the Teacher and ClassRoom streams need to have the name of a department. The ERD represents these relationships using a line that joins the SchoolDept stream to the ClassRoom stream and to the Teacher stream ( as shown in screenshot below).

When you create screens that show data, PhixFlow uses the relationships from the ERD to provide options for displaying data from related attributes in the same grid, form, card, graph or chart; see Compiling Views.

When you create a relationship from a primary key to an attribute, PhixFlow automatically sets the attribute to be a foreign key.  Foreign keys can have different names to the primary key, but they must have the same data type and properties. For example in the illustration above, the Teacher.Department attribute contains the same values as SchoolDept.UID; as shown in the attribute properties on the right. However, in the Teacher stream there may be more than one instance of a value. For example, the Maths Department UID could occur 10 times, once for each of the 10 maths teachers.

Although the relationship is between the primary key and a foreign key, when the foreign key is included in a screen, PhixFlow shows the attribute that has Display Name selected, which is the Name attribute by default. This gives the user a the meaningful string, rather than the primary key's identifier. For example, a stream showing Teacher.Department will have data such as Arts or Science, rather than an ID like1490.

Attribute properties for:

   ClassRoom
   Department

One-to-many and Many-to-one

PhixFlow always draws a relationship line between a primary key and a foreign key. This represents a 1:many relationship. The "many" end of the line has several lines. For example, one teacher runs many courses.  A many:1 relationship is implied when you read a relationship in the opposite direction. For example, a department has many teachers, and several teachers work for one department.

Many-to-many 

To create a many-to-many relationship, you need an intermediate stream that has foreign key attributes from each of the streams you want to have a relationship. For example, many students take many courses. A many-to-many relationship is shown below, using the intermediate stream called CourseAttendee. Notice that CourseAttendee does not need a unique identifier.

• A course is attended by many students
• A student takes many courses.

Using ERDs in Views

ERD's are important because the streams, attributes and relationships define the logical structure of the data that your application uses. PhixFlow displays this data to application users via views. Most commonly this will be a grid view, which displays data in rows and columns.

The concept for a view is described below and illustrated on the right:

1. Start with a specific stream, which is the base stream for the view.
2. Select the attributes from the base stream that you want to be the columns in the grid.

3. From related streams, select other attributes to add more columns to the grid.

   PhixFlow can only display data from a related primary key.

4. PhixFlow joins the related streams via their relationships.
   Under-the-hood this is done using a direct SQL join.

Link to how to create a view. I feel as though this needs the attribute selector

Why You Cannot Show Data from 1:many Attributes

If you add an attribute from a stream that is related to the base stream via a 1:many relationship, PhixFlow does not show any data in the grid rows. This is because there may be many values, which cannot be displayed in a single cell of the grid.

I am here - the following section should probably go .

Displaying Names Not Identifiers

Streams are related via primary keys. If a stream has a foreign key attribute and you use it in a grid view, the column displays the identifier. You can see this in the Teacher column below.

This can be put into the compiling views bit maybe

When creating the view, you need to choose a stream to be the base stream. Your choice depends on the records that you want to show, as it is the base stream's records that are displayed. In this case I want the records from the course stream, in which there is one record for each course, so when I create the view I start with this stream.

The left of the Attribute Selector / View Editor window lists related streams. By selecting the Teacher stream, I can add the Department attribute to the grid view.

For details of creating views and using the attribute selector see Compiling Views and Showing Data on a Screen.

Editing Data in a View

When you create a view that includes attributes from related streams, only data from the base stream is editable. For example, using the view in the example above, you could create a form to add a new course, with fields for:

• Department
• Course
• Classroom
• Teacher.

However, as the Department attribute is not present in the base Course stream, you cannot add the department information via the form.

Creating PhixFlow Streams From Existing Data

You can create streams in an ERD, but this is time consuming. If you already have data, PhixFlow can create streams from it.

If you are working with tables that have records, to see the records:

1. In the ERD, click on a table to open its properties.
2. In the properties toolbar, click  More Options.
3. Select  Show view.
4. PhixFlow displays a the default grid view, listing all the records.